A 450w power supply isn't enough to run the top of the line graphics cards. In fact I've put in a graphics card before without playing attention to the fact that you needed a higher internal power supply; and when the computer turned on, it was blank.

I'd suggest a 1000+ watt internal power supply if you plan to put one or more of the worlds most powerful Graphics Cards in your system.

Worlds most powerful Graphics Card to date = ATI Radion HD 3870 X2

Here's what I'd go for.

http://www.cyberpowerpc.com/system/Gamer_Ultra_8200/

Core Components

NZXT Zero Aluminum Full Tower 420W Case (I like full towers)
Extra Case Cooling Fan (2x)
Thermaltake ToughPower 1,000 Watts Power Supply - Quad SLI Ready (If not higher)
AMD Phenom™ X4 9850 Black Edition Quad-Core
(Take all of the freebies)
CoolerMaster Hyper TX2 Gaming CPU Cooling Fan
MSI K9A2 CF-F AMD 790X CrossFire Chipset DDR2/1066 Dual 16X PCIE (If not a better one)
I've personally never had much of an issues with memory so anywhere from 3GB of ram to 4GB is fine.
ATI Radeon HD 3870 PCI-E x16 512MB (If you want Super Gaming then go for the X2 version)
At least a 500GB HDD.


Accessories

(You do whatever you want here)


Software & Services

Microsoft® Windows® XP Professional w/ Service Pack 2
^You will need that if you want a gaming computer, Vista will slow you down FAR too much.^
(Set whatever else you want)

Final Price = $1246.00

Not shabby at all for a powerful computer.