Cheapest ive seen a 640gts was 217. 2900 is slightly better than a gts now but sucks up a lot more power. I honstly think you're better snagging a gts and getting them to install it. 320mb imo would limit you a good bit on some games with demanding texture requirements.
Gonna be a bit over the 500 mark but its a good pc for the price. Just make sure its a proper core 2 duo, E-6850.
http://www.overclockers.co.uk/showprodu ... subcat=793