Benchmarks from CPU-World show these two to be pretty similar, are you going to overclock? Honestly a 2500k would be best with a beefy HD6850 or 560 ti graphics card.
If you are going to be upgrading a motherboard for a new processor get yourself an inexpensive P67 motherboard and a 2500k (great for gaming) DDR3 is very cheap now if you don't have it and a strong graphics card is NECESSARY.
The Q8300 and II X4 645 is pretty even in most benchmarks. But if you're going to get one and is on a budget and can't afford at least a Phenom II X4 955BE then get the X4 with an AM3 + motherboard so you can plug in a better performing Bulldozer processor later on.