What components should I get?

[Your Computer]

As the the RAM, would the 4GB Corsair Vengeance Black Dual Channel Kit be good enough instead of what you recommended before?

Though it is slower, it would work fine.

I'm not planning on overclocking anytime either so I downgraded to the Coolermaster HAF 912 Plus Midi - £63.92. Would that be good enough or unneccessary still?

It'd be fine for the motherboards i recommended, but i have no idea if the CPU H&F i recommended would fit in that case. The stock H&F that comes with the CPU tends to be really loud, and you asked for a quiet system. The bigger the fan, the less noise.

I also had to swap to a PSU 600W OCZ ModXStream Pro Modular Silent SLI/Crossfire Ready ATX2.2 - £59.99. Let me know if that is okay... There is a 650W Corsair available but is around £70.

It should be fine, but if possible, go with the Corsair one. Corsair PSUs tend to be of higher quality and more efficient.

One last thing.... Will I need to buy a sound card or will the monitor have some sort of built in one?

Monitors, as far as i know, don't come with sound cards, but some have built-in speakers. The motherboard is the one with the on-board sound card. Now-a-days, motherboards come with HD audio sound cards, so you don't have to worry about a new sound card, unless perhaps you're an audiophile. Also, on-board sound cards tend to be better for recording.

Oh yeah I also forgot to mention about the CPU. Apparently others have been having trouble with Bulldozer for some reasons, so Phenom X4 II would be a more reliable CPU? Saying that though, the warranty is probably 3 years.

I think the issue is more motherboard related than the CPU itself. A lot of motherboard companies rushed their motherboards out to market with slightly faulty BIOS, but i can't say that is still the issue. I do know the Bulldozer turned out to be not as everyone thought it would be, but i can't say there's been an issue that i've seen that would prevent me from recommending it. Nevertheless, if you decide on the Phenom II, it would work on the AM3+ motherboard, you'd just have slower HyperTransport speed.