Well as has been said the 7900 is a major step up from the 9800Pro
To give you an idea I've just run the HardwareOC Prey benchmark on my specs
<----My system
1280x1024, maximum on all settings, the resault is 52.6 FPS
So yes I'd say a 7900GS is a good card esp for 1280x1024.
I also ran the timedemo on Halo1 = Average frame rate=75.47fps, it may look a little dated but it's still fun and boys does it speed along on these specs
A 7900GS or 7950GT is a great buy
2nd sugestion, now I'm going to rock the boat a bit here and also sugest a factory overclocked 8600gt & an SLi motherboard, like
256MB MSI 8600GT Overclocked @£72.60 or
256MB Leadtek 8600GT Extreme @£85.76 (note the MSI is cheaper because it's GDDR2 where as the leadtech is GDDR3)
My reason is, that it's around the same price as a 7900GS, granted not as good performing but it's good enough at 1280x1024. But the real key here is SLi, the 8600GT does extremely well in SLi
http://www.xsreviews.co.uk/reviews/g...si-8600gt-sli/ seems to be down at time of posting 
Basically with the latest drivers the 8600gt SLi is just behind the 8800gts 320mb and I'm working on the idea of a low cost spend now with the option of a 2nd low cost spend later to spread the cost out, also has the ability to do DirectX10 which the 7 series don't. (How well a single or SLi 8600gt will be in DX10 is still up in the air, but by the time a few dx10 games are out you'll know if it's worth SLi'ing it or getting something better)
3rd sudgestion, if you can scrape the cash together a low cost 8800GTS 320mb
Novatech GeForce 8800GTS 320MB @ £172.72 that's one of the best prices I've seen for a 8800gts and come with a "lifetime" warranty from novatech.
4th option is a x1950Pro or x1950XT, generally faster than 7900GS or 7950GT, however they do underperform in OpenGL (the x1950Pro is comeing out just under the 7900GS in OpenGL based games and most older games are OpenGL based) I also still see a lot of threads about x1950 cards having problems esp with stock coolers (there were 7900GT problems but they seem mainly to of effected eairly cards, I haven't seen many problemmatic 7900 threads recently)