What is the best bang for my buck I can get with $250?

Right now I am building a computer. I already have a decently fast HDD, a case, CD-DVD ROM drive, etc. but I need to up this computer to date for under 250 bucks. The motherboard only supports PCI and 2GB of DDR PC3200 800MHz, the FSB is only 800MHz, and the processor is a Pentium 4 @ 3.0GHz. What I've done is I have gone on Newegg and searched out what I think is the best performance to cost ratio'd combination of CPU, GPU, PSU, Motherboard, and RAM.

EVGA GeForce GTX 460 1GB 256-bit GDDR5
AMD Athlon II X2 3.1 GHz
Sunbeam PSU-BKS580-US 580W ATX
4GB DDR3 1333MHz SDRAM
BIOSTAR A770E3 AM3 AMD 770 ATX AMD Motherboard

This all totals up to $276.95 including tax + shipping.

The motherboard supports PCI Express 2.0 x16, FSB is 2600MHz, it has 4 x 240 pin slots (up to 16GB of DDR3 SDRAM), and an AM3 slot.

The main things I'm worried about are the power supply not being enough for the rig.

Also, it would be much appreciated if you could recommend me some other combinations under 250 with about the same performance, because I want to be able to play Battlefield 3 when it comes out near highest settings.

Oh, and BTW, I play at 1024 x 768 resolutions, I might upgrade to 1280 x 720 later on though.