You will see a noticable difference in that upgrade. Definitely worth upgrading and I think your cpu is bottlenecking that card. Do you have your cpu overclocked? If not I would do so and run 3dmark06 by futuremark before and after the cpu overclock to see the results.
I have a 8800GTS 512 OC GPU.
I had a 4600X2, socket 939, DFI Lanparty, and 4 gig of DDR Memory.
I bought a Gigabyte board, a Phenom II 920, and 4 gig of Crucial DDR2 memory.
Bought it on sale at Frys back at the first of the year, spend $295.
Put my 8800GTS back in.
My 3dMark 06 score with the 4600X2 was around 9500, could hit 10,000 with a little overclocking.
My 3dMark 06 score now with the Phenom II is 14,300, can hit 15,200 with a little overclocking.
Big difference, and yes you can tell it.