I guess I may be a little biased, but I personally think its better to build your own system - you always seem to get screwed on something when you go premade...but I guess that could just stem from the fact that until the first computer I built when I was 14 or 15, I was stuck with stuff from the likes of Packard Bell Anyway, I think a decent system can be had for $1000 plus case and mods....all prices are at newegg unless otherwise noted...
Asus A8V Deluxe - $120 (zipzoomfly.com)
Athlon64 3000+ boxed - $146
1GB Corsair ValueSelect pc3200 ram - $87
MSI GeForce 6600GT - $200
Seagate 120gb 7200rpm SATA HD - $97
Sony Dual Layer DVD-burner - $56
Logitech cordless keyboard/mouse - $58 (zipzoomfly.com)
19" samsung CRT monitor - $180 (frys electronics - don't buy a crt online)
Windows XP Home OEM - $94
The pride and satisfaction of building your own machine - priceless
Total - $1038
Left for your choice of case and modding supplies - $262
You figure a nice case is $150, you still have $110 left over for lights, fans, plexi, paint, etc. Now if you wanted to do a full-on complete case makeover like CrimsonSky, etc do, that might be a different story...
Honestly, I think that's a pretty good deal on a nice system. Can you get a prebuilt system for cheaper? Of course, but then you lose some of this upgradability and they always seem to skimp on at least 1 thing (that and the coolness factor of building your own system is a plus). Better deals on the parts I listed can be found, and hell, you could go with some cheaper hardware if you felt like it as well. By no means, am I saying, "This is what you should get. Do this or else." I'm simply offering an alternative suggestion to you in response to those who say you should get a premade system. Do you have to listen to me? Hell no, its your money afterall. And also, I don't know what your current financial situation is, but don't be afraid to throw a little extra of your own cash in the whole deal if you can - it never hurts
This is just my $0.02, that's all. If you don't like my advice, please give me my $0.02 back; I'm a little short on cash right now