regardless of whether you budget is large or small building your own computer ALWAYS saves you money! The cheapest Dell (no monitor or speakers... nothing but the box) was $349, on newegg i was able to build one for $327
https://secure.newegg.com/NewVersion/WishList/TemporaryWishList.aspincluding a $10 MIR. So while it's cheaper, it would only make sense to build your own if you already knew what you were doing. Honestly though, If someone told me they wanted a $350 dollar computer... I would point them towards the Asus EEE ultraportable, At least it's a complete system! The EEE might even be more powerful as well (celeron 420 in the mentioned Dell)!
If your price range is $750 or more, building your own becomes an even greater bargain. Dell is known for using absolute junk when it comes to motherboards, PSU's and HDD's. The combination of all these junk components paired with the bloatware will make your machine clumsy, sluggish and maybe even BSOD prone. I've seen a lot of cheap PSU's and Mobo's cause a lot of problems in computers. When you build your own computer you have much greater quality control. I built a barton 2500+ system almost 6 years ago and it's still going strong on my fathers desk! I've heard of Dells arriving DOA! On top of the higher quality is getting what you want! Plan your upgrade path and pick your components well, and you could be using the same case, HDD, PSU, etc. for a long, long time saving you hundreds of bucks 2 years from now!
If you decide to build your own you will be forced to learn and research some information that will be very useful down the road when troubleshooting problems that might arise. Keep in mind that if you simply cannot operate a screwdriver, building a computer isn't for you (neither is using a stove or riding a bike... in fact, you might want to invest in a sturdy helmet). I think everyone should at least know how to reinstall or repair a windows installation! I know it's hard to put the dvd in the drive, enter your key and wait... seriously? As for alienware, well, my system cost over 25% less than their $2,000 equivalent. Not to mention high quality components that I'm confident will last well into my next build as well as the ability to overclock my system (thank you abit and zalman 9700!). Also, alienware computers look more like fancy coffe makers than computers... I much prefer my antec p182 thank you.
so heres the list:
1. You
always get a better machine for your money or same machine for less.
2. All part's are known quantities.
3. Gain invaluable experience (we're all power levelers at heart).
4. Dell parts are cheap/junk/poop.
5. Dude, your not gettin' a Dell!
6. It's not as difficult as alienware says (remember to tighten the chinstrap on your helmet).
7. It's very rewarding and satisfying... to bash iTards when you find out you have the same computer but it cost 50% less!
... my $0.02. (computer dork manifesto)