What resolution are you gaming at? If you're gaming at 1680X1050 and higher, then you would probably notice a decent difference with the 4850. If gaming at 1440x990 and lower, you probably wouldn't notice much if any difference. I just recently upgraded from a 8800gts 320 to a 4850 (and then 4850 xfire) and although there was a noticeable increase in FPS, it wasn't earth shattering like I was hoping for.
Believe it or not your video card is still a pretty decent card. Had I to choose however, go with the 4850.
Also, in my opinion, anything higher than a 4850 with your current rig would most likely be bottlenecked by your CPU.
hmmm....Well a 9800gtx really isn't that much more powerful then a 8800gts 640mb. Just like the above poster said...if you are playing at resolutions 1680x1050 or even 1920x1200 then upgrading wont do you any difference, becuase your card is still considered towards the high end and is very powerful. Infact at 1680x1050 your card can easily max out any game on the market except crysis
I have/had a 640 card. I also have a 22" monitor...16x10. If you can't upgrade to a GTX260 don't spend your money. The 640 is getting a little long in the tooth but still has it's uses and going to a 4850 won't be much of anything TBH.
To see very large gains in FPS, you probably need a GTX 260 Core 216 (or GTX 280) or the HD4870 or higher ATI wise.
My 8800GTS (512MB) does really well with current titles at my 19" monitor's max 1440x900. After looking at benchmarks on the newer cards, I've realized it's not worth upgrading for me yet.
Now, with a 22" monitor, you'll be doing 1680x1050 or higher depending on your monitor. You will probably see some gains in performance that way, but don't expect anything earth shattering.
Check out the VGA charts here on Tom's Hardware and compare different game titles @ 1680x1050 with your video card compared to the ones you're tempted to buy.
That's what kept me from upgrading frankly. I almost ordered a second 8800GTS (512) a couple days ago to do SLI, but at my 1440x900 there really was hardly ANY increase in FPS possible at such a low resolution. Short of buying myself a much bigger monitor, it wasn't worth any upgrade.
EDIT: Just for my $0.02. I'd upgrade RAM and CPU before you do your video card. If you put in one of the GTX 260/280, ATI 4870 or higher, your CPU is going to create a bottleneck and limit your video card's performance anyhow.