Tough call I love EVGA personally, if they made ATI cards I'd most likely go insane trying to make this choice lol. But I've had a few EVGA cards and if anything happens their support is the best (just make sure you register your product when you buy it)
The 4870 1GB is by no means a bad card though.
So it really comes down to the card that performs the best in the games you play and the support from the company you purchase the card from. Also if you do plan on SLI/Xfire in the future making sure your Mobo is compatible. So look up some charts with the 1GB 4870 and the GTX 260 core 216.
Also a note on the GTX 260 to make sure you pick the best possible one:
The HD 4870 1GB offers only a small increment of additional performance compared to the regular 512MB version including at 1920 x 1200. If spending the extra cash for an additional 1 - 3 extra FPS on average is enough then go for it.
As stated above, EVGA has the best warranty and support. As far as the games you listed, it will look like this:
Crysis/Crysis Warhead - GTX 260 wins (Although I've seen benchmarks to the contrary, most of the time the 260 comes out on topSee here
Half-Life 2/anything else on Source engine - 4870 wins (2nd graph
Guild Wars/RTS - Can't find benchmarks on Guild Wars/most RTSs, but it will be a pretty even draw.
(Not sure if it's an RTS, but here's some benchies on Brothers in Arms here)
The EVGA GTX 260's pass audio through HDMI via internal SPDIF as well.
Most of those benchmarks are for 1680x1050, so at 1440x900 you'll see a bit of a jump. And of course *disclaimer* your mileage will vary depending on the rest of your system.
You didn't mention this-
Are there differences between the cards with respect to the output connectors or any other feature? These kinds of things might be important to some people, not important to others.
I've noticed in reading the reviews on newegg that some brands get better reviews than others. In particular MSI seemed to uniformly get better reviews than the other brands. This might give you some more input to consider.
Your board is setup to handle crossfire so why limit yourself on future upgrades by buying the 260. This way if you get the 4870, you can wait another month or two and purchase another 4870 when the prices drop again after the new cards coming out most likely in April.
+1 for what croc said, you don't need a 4870 or the 260 the res you game at the 4850 is enough
Yes I use that resolution and card (4850) but i still recommend the 4870. I can max out most everything but he's gonna be MUCH happier with the 4870. The 512mb version should be good enough for you man, just look for a good deal/price drop before you buy. Don't pay more than $150 USD for one.