I upgraded my nans pc from a duel core c2d to a q8400, 120gb SSD, and 6gb ram, (s 775 CPU's and ddr2 ram is dirt cheap from that highstreet second hand shop).
Ran a bit hot with the stock Intel cooler so had to spend £20 ish on a decent cooler.
It runs fine. But the upgrade cost about a hundred quid. It might have been better to buy an old i5 ddr3 bundle.
If it's just for general desktop use, I'd probably in hindsight just put a cheap 120gb SSD in it and put more ram in it as ddr2 ram is cheap as chips.
The q8400 was only about £12, but i had to buy a better cooler to cope with it, so the CPU upgrade ended up costing like £35
You are still limited by the hardware though on a s775 platform, it's absolutely fine for a media/surfing /office box but it's not much cop for gaming unless your happy playing older games or indie games.
There's not much point upgrading the motherboard if you're current one works.