Crossbow:
The only game that'll suffer with just about any card & res at higher settings is probably Crysis.
That's cos it's poorly coded!!
The 4870 is a great card and will handle all if not most of the newer games on high detail. May struggle a little at 1920 x 1200.
I'm using a 4870x2 but only at 1680 x 1050. Not sure about the 4870 single card, but if you have the x2 you don't need any central heating in the house.
Note also, it's worth checking to see if your power supply is upto spec before buying.