Your 8800 GTX is the maximum card your CPU can make use of. Many of your games will completely max out the CPU (at least one core; most games can't use two cores fully, that still means the CPU is maxed out).
You can look at this chart and get one at or below the 8800GTX level but you may wish to get another 8800 GTX.
http://www.tomshardware.com/reviews/geforce-gtx-radeon,2326-8.html
so the 260 would be a waste of money.
Your other option is to purchase a faster CPU but at that point I'd be considering a new computer. I'd also upgrade to 4GB of RAM if it's not too expensive.
You are actually quite crippled for many newer games by having that CPU. I have the X2-4800+ and compare CPU usage (CTRL-ALT-DEL) so I know what I'm talking about. I've maxed out the CPU on at least one game using only an HD3870. I estimated the HD4770 would max out most games on the CPU but not all so it would be optimal. At this point I'll just wait though and upgrade the whole system.
Chart:
I noticed that this doesn't have all cards. The 8800 GTX may also not be the best deal. Anyway, use this as a guideline. Remember, a more expensive and faster card will work but you won't see any benefit in games.
If it was me, I'd upgrade your system like this:
- AMD X3 720
- 4GB (total) RAM (DDR2 I assume)
- GTX 275 896MB