You shouldn't be looking at prebuilts. You'll pay extra and get lower quality parts. I'm not in the UK, so I don't know where is the best to buy from. I have seen a lot of preferences for Ebuyer though.
Here's what I would look for (links/prices in US only):
Any quality 650W unit will be able to Crossfire the 5850. You'll likely need adapters to get the right connectors, but the PSUs will be able to handle the load.
And by cheaper, I meant the cheapest of those brands listed. Look at the offerings from those companies and get the cheapest 650W unit in that group. You'll get a high quality unit at a lower price that way.
My uncle is telling me a Quad Core processor is needed for newer games. Is he right?
Almost no games require a quad core. I actually can't think of a single one that has a quad core as a requirement. There are a few that don't run well without four cores (like the GTA), but those aren't all that common either.
Triple cores are actually better for gaming overall. Search the articles here at Tom's for one called "How many cores do you actually need" (or something along that line). There's a review they did to compare actual gaming performance with a single, dual, triple and quad core. It found that adding the second and third cores had a huge performance gain, but adding the fourth core actually decreased gaming performance slightly.
Fair enough on the PSU. I know OCZ is really cheap over here, but they're higher quality units (the ones with high efficiencies) are more expensive than the lower efficiency units from those companies at the smaller sizes. It was just something to check out.
I've always been a big GTA fan, and would like to be able to play it on my new PC. Should I bump up the CPU (I can probably stretch the price) to a X4 630 Quad-Core?