Graphics card for AMD Athlon X2 6400+

Currently I'm debating on upgrading my old 8600GT card. My rig, however, is other than fresh. It's AMD Athlon X2 6400+, 4GB RAM, Asus M2N-SLI mobo, WIndows XP. Money is tight now and upgrading CPU and/or motherboard is not an option. Only want to upgrade my video card. I tried looking up some reviews already but all video cards are being tested with the top of the line gear, and mine isn't. Want to play COD4/5, Far Cry 2, Crysis, Diablo 3, Starcarft 2 (whenever they come out, lol) I know, it will not be as great as on i7 system, however would like to squeeze everyting I can out of my good ol' rig. So, I've been thinking, what should I get:

1. 260 GTX 216
2. 4870
3. Wait/save more and get GTX 285 or 4890
4. 260 GTX SLI ??

Power supply is adequate for any build. Main questions to figure out is if my CPU going to hold up ok, meaning if I'm getting the bang for my buck. It probably should be fine with 260/4870, but I think it doesn't have enough juice for 285 or 260 SLI. So, what would be better, faster and value/performance tight in my case? If I aim low end, what's better 4870 or GTX260 for my rig? If I pay extra for 285 or 260 SLI, is the difference gonna be worth it?

All comments are welcome. Looking for some experience folks' opinions.

P.S. Yeah, my CPU sucks, but I'm going to have to live with it. Due to money constraints, CPU/Mobo upgrade is out of question.

I have a 24" monitor and hope on being able to play comfortably at 1920x1080 (1080P is native screen resolution).

The reason I used word "adequate" is because I will upgrade to what's needed once the video card is decided upon. Therefore, I've made an assumption it's irrelevant in this case.

you could pick up a pair of 9800GTX cards pretty cheap and SLI them.

btw, my 6000+ is still doing it for me

Yeah, GTX 260+ or an HD4870 1 gig sound about right for that resolution without your CPU being too much of a limiting factor.

You're CPU isn't too bad, though it would do better if it was overclocked. A 260 sp 216 would be a good choice. I can't imagine another one in SLI would do you much good without a really good CPU overclock. You may want to go with the 4870 1GB board though if you'd rather get a crossfire system in the future. After all, SLI ads a few more bucks to an i7 build or if you want to go AMD there aren't any AM3 SLI boards (no the Asus 980a board doesn't count because it uses DDR2 Ram). Crossifre is also more flexible as you can crossfire a 4870 with another 4870 or even a 4890 latter, but it's always a bit more, interesting, when mixing cards like that .

1gb 4870

You're going to see a massive improvement over the 8600 GT. Congrats!