What resolution do you play at?
right now factory OC GTX 460s (768mb) can be had for about $150 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
You won't see too much of an issue from the reduce memory buffer, or bandwidth, unless you play at higher res with lot of AA.
If you want you could get an HD 6850 for a bit more, or a factory OC GTX 460 1GB, these might fit in with rebates.
go for a HD6850 at $179.99 then. Its a little more than you wanted, but you can also run one of them with an Antec EarthWatts 380W PSU with room to spare. a 430W If you are worrid about it. 550W is enough to crossfire.