I was in a similar position last week but I had the equivalent of $600 to spend and I also had 2x G8800GTS 640's in my system. I had the X1950XTX and when I upgraded to first 88GTS it was unbelievable. Adding the 2nd wow!
However, over time, SLI and driver issues/ profiles started to bug me and I got the $600 bonus and decided to go back to one card config. I looked at the 4870 and also the upcoming 4870X2 and as I'm going to the states in a few weeks thought I might bag a bargain.
NV then dropped the price of the GTX260 to £180 and as I had someone lined up to by my cards at £140 I jumped. What put me off the 4870 were some of the reviews mentioning heat and noise and also the fact that it's always best to reformat the disk if your moving from one IHV to another. I couldn't be bothered.
The GTX260 is better than my SLI config with lots of AA & AF @1680x1050. Overall a happy chappy I jumped.
Asus P5N32-E-SLI mobo, Intel Core 2 Quad 6600 at 2.7, Radeon HD5850 1GB, HP 22" LCD @ 1680x1050, 4Gb AData 800 mhtz ram, 1TB Samsung Spinpoint F1 & 250gb Seagate H/D, 2x DVD Burners, X-FI Xtreme Gamer, Thermaltake ToughPower 700w PSU