In 5 years time the PC will be barely worth using for flash games on the internet, it is just the way technology is. If you continuly upgrade gradually then you can be on top of the games requirements.
This should fit nicely in that budget:
AMD 64 3500 venice
DFI lanparty NF4 SLI-D
1gb OCZ platinum rev. 2 (2X 512)
Seagate/Western Digital hdd
nice case with good air flow
Artic Silver 5
Overclock this nice and high - while maintaining a low temperature, that will last you a couple of years. Then get a new graphics card with some money you save, save up again, get a new motherboard and CPU, then upgrade the rest as you get the money.
Cisco CCNA, Comptia A+, 1/3 through CCNP