oh i think a 2.8ghz c2d would definitely be fine with an 8800gts. personally i dont think its worth the hassle or cost of upgrading. plus you dont even know if your processor can clock that high; for all you know 2.8ghz is it's limit, and if that were the case you would just be wasting your time. you got a free 1ghz speed bump out of it, just be happy with that. what you could do to see what kind of difference you'd see is run some benchmarks (or whatever you want to compare) on the processor at stock, then overclock it by 700mhz, run them again, and compare the results. since the performance increase is pretty much linear on these chips, you would see a pretty similar difference in stock > 700mhz above stock as you would with 2.8ghz>3.5ghz. if you really wanted to get a good analysis you could also clock it at 2.1ghz and see how that compares to 2.8, or if you really really really want to see you could take results at 100mhz increments and see if you can find the rate at which it increases with respect to the clock speed, derive an equation, and plug 3.5ghz into said equation to see approximately how well it'd run. of course, this is if you really really want to find out. just testing 2 that are 700mhz apart would probably be fine, although if you wanted to be thorough more testing at different speeds would be the way to go.
i hope that made sense.