A dying Cards replacement

I've got an Nvidia 7900GT 256Mb card that is pretty much on its last legs. Having said that its been on its last legs for quite a while, I just didnt have the heart (money) to replace it.
Finally we have the power, we have the have the technology, but we dont want to spend alot of money.
I'm not sure what would be the best option for me to upgrade to though, since I would love to grab a HD4670, but I dont think that the rest of my system can really keep up with it.
My specs that matter are as follows:

S939 AMD 3800+X2
2Gb RAM DDR400
Foxconn motherboard of some kind(I can't remember and I'm not at my primary, though it does have SLI)
Monitor is 19" 1440x900

I cant be bothered to add the hdd model or anything else, but ask and ye shall recieve.

Anyway, in regard to my CPU and screen size, is it worth to go for the 4670, or stick with the 4650, or go with something else?

I prob. should have mentioned that a limit of about 60 pounds applies, which pretty much limits me to a 4670 or under. As much as I want to buy a 4870 or 50, I'm not likely to play above 1440 for the next 3 years and I doubt that I would upgrade my system till DDR3 becomes really affordable and maybe when AM4 (or equivilent) will be released, in other words another 3 or 4 generations away

A 4650 is a couple of levels better than the 7900GT. About the same as a 8800GT-256 or 8800GTS-320.
At your 1440 resolution, the cpu may well be a limiting factor. What does the task manager say about cpu utilization while gaming?

Is your 7900GT failing in the hardware sense, or is it no longer failing to perform adequately for the games you play?

My mates running on a HD4670 and hes well happy with it for what we paid and the performance he got and to be honest the cards probs slight overkill for you but hey you'd rather overpreform then under right? and when ever you do ungrade in the future instead of buying a new GPU you have the option of buying antoher one or two hd4670 and running in crossfire (if mobo can etc...) anyways i'm buying two HD4850's soon and new mobo OC my processor and running crossfire (thinking about buying a third in a month or so... when my credit crunch is over =P) anyways good luck!

Since you mentioned a budget in pounds, I did a quick search at a UK site:

HIS ATI Radeon HD 4850 512MB 123 pounds, 800 stream processors
http://www.overclockers.co.uk/show [...] =GX-099-HT

HIS ATI Radeon HD 4830 512MB 103 pounds, 640 stream processors
http://www.overclockers.co.uk/show [...] =GX-107-HT

HIS ATI Radeon HD 4670 512MB 66 pounds, 320 stream processors
http://www.overclockers.co.uk/show [...] =GX-103-HT

As you can see, the HD 4850 is better bang for the buck than the HD 4830, which is better bang for the buck than the HD 4670. If you can afford the HD 4850 and your PSU can handle it then get the HD 4850.

@OmegaStalker:
I meant the OP. My bad, I should have put an @OP at the beginning.

But that's OK, congratulations for your new PSU. It's a very good one.
No idea about xpertvision, sorry. I don't think they sell anything in North America. Not at Newegg anyway, I looked. I'm guessing you want a pair of nVidia cards then, since xpertvision makes that kind of cards. That PSU can handle two 9800GTX+ cards all right. It can handle two GTX 260 Core 216 cards if absolutely necessary, but with lower efficiency, more heat, more noise, and only if you keep the number of disks/fans/lights/PCI cards low. For nVidia cards, I trust eVGA, BFG and XFX most.
What kind of MB did you get?

hmm, somehow I think that this thread is starting to get hiJacked. Anyway the PSU is an arctic power 500watt. It seems good, but feel free to say otherwise.
@aevm:
I my realistic limit is 60 pounds, so its a choice of 46xx cards, or 96xx cards. Though 46xx as better bang for buck compared to 96xx.