I have to disagree with jenspetersen. (But let's not make this a flame war, boys and girls!)
There really isn't that much difference in speed between an 800MHz processor and a 1000Mhz processor ... not in real-world applications. The Thunderbird will beat the P3 at that clock speed in several (perhaps most) benchmarks, but benchmarks don't mean very much except for bragging rights. And if you wish to overclock, it's probably easier to overclock the P3 than an Athlon, mostly because the P3 is a cooler-running processor.
Either one will be more than sufficient to run almost any desktop application you care to install, including games.
The real deal is ... you've got a limited amount of cash to spend, and I can also see that you wish to upgrade with a minimum of fuss and bother. Neither one of these platforms leaves much room for upgrading in the future, due to the Slot/Socket, so that's not really an issue. If you want the ability to upgrade in the future, you should save your money and purchase an AMD/DDR platform, which would be your cheapest and most viable option from that viewpoint, currently ... or wait for the P4/DDR.
You said yourself that it will be cheaper to just buy the P3 than to purchase the Athlon and a new motherboard. So that's the way I would go if I was strapped for cash.
Here's an interesting article for you to read that might help you decide: <A HREF="http://www.anandtech.com/showdoc.html?i=1252&p=1" target="_new">http://www.anandtech.com/showdoc.html?i=1252&p=1</A>
You'll see that the Athlon is the superior processor in several respects ... but not by much. You'll just have to judge for yourself if getting one is worth the extra money. Personally, I think that if you set two computers side-by-side, one with the Athlon 1GHz, and the other with, say, an 866Mhz P3 ... you'd be hard-pressed to see any difference in speed with the naked eye.
Toejam31
<font color=purple>My Rig:</font color=purple> <A HREF="http://www.anandtech.com/mysystemrig.html?rigid=6847" target="_new">http://www.anandtech.com/mysystemrig.html?rigid=6847</A>