what you can tell me is how much less will it cost you compared to going with newer parts, let's say an i3 3220 + H61 board, or Athlon X4 750K (if you can find one), FX 6300 or something...
as for e8600 vs i3 3220, the i3 is clearly faster, it have higher single core performance and HT helps considerably for a few games.
this should give you an idea, even at 4GHz the e8xxx is still slower than the 322x (the 5 have a faster IGP, but for this test the IGP was disabled, so performance is the same to the 3220)
http://www.tomshardware.com/reviews/ivy-bridge-wolfdale-yorkfield-comparison,3487-20.html
if it's a good upgrade, I don't know, normally I would say going from the e8600 to some i5 would make more sense, but...
now the i3 540 is not as fast, but it can be overclock a lot more (compared to the 3220), and the e5500 is significantly slower than the e8400 (1/3 of the l2 cache, which used to make a considerable difference, to match the e8400 stock performance you would need the 5500 working at around 600-700MHz more I think)
anyway, I think this represents well enough the e5500 (same cpu basically only the FSB is lower for the e5500) vs i3 540
http://anandtech.com/bench/Product/94?vs=143
not a huge gain, but I would expect the improvement to be better on newer games