Advice on a computer I'm building

Hello masters of the technical and electronical. I am planning on building new computer, one that is primarily tilted towards gaming, and I wanted to run the parts by you all to see whether or not I was making some terrible terrible mistake on something or whether everything was good.
The goal in builiding this franken-computer, was to build a machine that could be easily upgraded a year or two down the road at minimal cost. So far, I have the following parts (my budget, by the way, is just around $750):

- XION XION II XON-103 Black SECC Steel ATX Mid Tower Computer Case with a 450W Power Supply included.

-ASUS A8N-SLI Deluxe Socket 939 NVIDIA nForce4 SLI ATX AMD Motherboard

- XFX PV-T71G-UDE7 GeForce 7900 GT EXTREME (520MHz) 256MB GDDR3 PCI Express x16 Video Card

- AMD Athlon 64 3500+ Venice 2000MHz HT Socket 939 Processor Model ADA3500BPBOX

- CORSAIR ValueSelect 1GB (2 x 512MB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered Dual Channel Kit System Memory Model VS1GBKIT400

The price is about $786 with shipping. I wanted the SLI motherboard so that, in time, I could buy another 7900GT and run the two in SLI mode. As for the ram, I could probably do a lot better, but i'm pretty maxed out on my budget.
I already have a monitor and 2 250GB hard drives from another computer that I'm going to use.
Any advice you could give me on this would be greatly appreciated - as to whether I'm doing something right or wrong. Thank you very much.

 

the PSUs that come with cases are usually cheap and not very reliable. for that system id get at least a 500watt PSU that is SLI Certified so you can be ready for that second 7900gt. Other than that, looks great. Good job, thats a great system for that price. O, just noticed sumthin....where is your hard drive and disk drives?.....maybe u already have them but just asking

 

Yeah, I already have some old 250 hds from my old desktop that I'm going to transfer over to the new one. If the power supplies really do suck that much, then I may as well get another one. I'll see if I can find a case/psu deal somewhere. Thanks for the advice!