newgen choices for Nvidia post 400$
670
680
690
newgen choices for amd post 400$
7970
7970 ghz
upcoming 7990(im not sure if powercolor will ever release their 7970x2)
the most expensive single gpu card would be the 7970 ghz, the 7990 and 690 are single part dual card gpus.
if this is in rs(indian currency), then thats a heckofa low budget