As to RAID1, I myself use it, and I couldn't imagine not having the redundancy. I've had too much lost from hard drive crashes. In a way it's costly because you're buying two drives for the storage capacity of only one drive, but it is the cheapest redundancy solution out there.
The performance however is nothing like RAID5. You basically get the performance of a single drive. (The read is a <i>little</i> faster than just one drive, but not by much, and the write speed is the same as a single drive.) So I'm not sure how impressed you'd be with RAID1 for speed if your current setup is too slow.
RAID 10 (there are a couple of ways to combine RAID 0 and 1, but they all seem to do about the same thing) performs noticably better than RAID1 because the RAID0 striping does boost it's performance. But it's still not as fast as RAID0 by itself, and again, the cost is that you're using four drives but only have the storage capacity of two, which can be expensive. By the sounds of it, this is probably the most fool proof solution for you in your price range. Because striping (RAID0) and mirroring (RAID1) are both very easy for a drive controller to do, an awful lot of cards and even motherboards will support this. (And even if they don't, Windows XP can do it in software, though I personally don't like relying on Windows for these kinds of things.) It's no where near as draining as the XOR or RAID5 is. So your only real cost is four drives and a case that can hold them. It should be easy to find a motherboard for you that'll do RAID10 without having to use Windows to do it.
However, as a programmer myself, I can identify with your need for speed. Back in the old days my projects used to take hours to do a full recompile, which of course makes debugging very ... painful. Things are much better today, but still, time is money. So you might want to consider something faster (and cheaper) than RAID10.
You might want to try a RAID0 array with two 10K SATA drives. Since RAID0 doesn't waste hard drive space (because it has no redundancy) two 36GB drives for a 72GB total would probably do. (But you could easily look into two 72GB drives if you needed the space.) Then get a large slower SATA or IDE drive for backups. I think Windows XP even comes with a backup utility. (Though personally I prefer using Norton Ghost myself since the latest version runs fine in Windows.) Set up your system to do automatic nightly (or whenever you're not on the computer) backups of the RAID0 array onto your large slow drive. This way you get great hard drive performance when you need it and almost as good of a redundancy system as RAID for a <i>lot</i> cheaper than RAID5. True, it's not quite as fool proof as RAID10, but it definately costs less and outperforms it, so long as you don't mind your computer having some downtime to do its backups. And if you're worried about virii you can even put your backup drive in an external USB instead of putting it in the PC directly so that it's not always connected.
And, of course, there's still software RAID5. If you're only interested in redundancy, software RAID5 is pretty cheap too. With three drives it'll perform noticably worse than RAID10, but a bit better than a single drive. With four drives software RAID5 performs <i>about</i> the same as RAID10. (Sometimes better, sometimes worse.) So it's not completely bad, but again, it's not as stunning as what you'd think when compared to a good SCSI RAID5 setup. But in your case with a lot of compiling taking up CPU resources, you might see software RAID5 perform noticably worse because your CPU is being heavily used. I'm really not sure how that'd turn out.
So the RAID10 solution is obviously easiest and least prone to human error. The software RAID5 solution should cost a little less, but in your case will probably perform hardly any better than a single drive would. That, of course, puts it on par with RAID1, which would be cheaper yet, but offer no performance benefits at all. And then there's the RAID0 + backup solution that would give the best performance out of all of the above, still have redundancy, and is at a reasonable price. It's just also definately the most fiddly way to do things and requires your PC to have regularly scheduled downtime.
Of course you could also do something like a RAID1 array of two 10K SATA drives (or even two 15K SCSI drives) for your coding, and have a seperate drive (or even 7200RPM RAID1 array) for everything else. Just splitting your hard drive usage so that the compiling is done on its own drives and controllers could improve your real-world performance noticably, especially with SCSI involved. Just a thought.
By the sounds of it you'd also benefit from a dual-CPU computer. Dual-core CPUs are the fun new cheap(ish) way to do that. I can't say that Intel's dual-core CPUs are very impressive yet though. AMD definately has the one-up there. So an Athlon64 X2 would probably help you out. If you do go dual-core or dual-CPU, the software RAID5 might look a little more attractive then as you'd have a bit of extra CPU to throw around at it.
And if you <i>really</i> want to get rid of your compile time, try switching to a nice interpreted language like Python. PyQt makes for some very nice GUIs you know.
😉
Actually, I almost miss the old compiling days. Having an excuse to goof off while your code compiled had its benefits. :O MUDs were killer ways to spend compile time.
And as for where to buy the system from, I partly agree with endyen that a local shop could be the best place to look for such a custom build. I always like to support the small businesses when I can. However, I've also been soured on that when it comes to PC shops lately as I must have moved into a den of f'ing rip-off artists that believe in 500% markups and even some flat out cons.
🙁 So if you do go local, go in armed with good research first and be prepared for disappointment, just in case.
Otherwise, I don't really know of a good online shop that'd build a more exotic setup. Hell, I don't even know of a good online shop that'd preconfigure a RAID array. :O Hopefully someone else knows of a good one if you go that route. Otherwise you might have to go at it yourself. Building a PC isn't really so hard these days and can be a fun learning experience, but you'll definately want to get an anti-static device if it's your first time and keep the old PC around to jot online for help when needed.

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<font color=red><i>Deal with the Devil. He buys in bulk.</i></font color=red>
@ 197K of 200K!