8800GT vs HD 4870 but with a constraint

The constraint is the fact that I have a Pentium D 940 and according to the information I can find about my motherboard it can only support this now obselete range of CPUs. This as it may be, would I see any significant increase in performance by upgrading my current 8800GT to something else (I am quite drawn to the possibility of getting a HD 4870)? CPU upgrades that require a change of mainboard is sadly out of the question even though it seems that the CPU is the main bottlneck at low resolutions (My native resolution is 1280 x 1024). Thanks for any thoughts,

R_N

Thanks. What if a new mainboard was not entirely out of the question (sorry to extend the question). Anything I can do for around £200-250 (or, even better, under this amount). I've been looking at the e5200 as an ok (and also quite cheap) CPU. Would this allow me to get better performance from my 8800GT? If so, could you suggest a suitable motherboard? Sorry to extent the thread but...

Thanks for anything else!

R_N