Help Me Build a New Computer?

Hey,
I would just like to say my computer is shit that is compared to all you guys
I am hearing that you guys can run CSS on I think I heard 100-600fps regularly whilst I can run it on 40fps regularly with some maps on 100 which is not great.

I really want to build my own computer, but I don't really know where to start.
My budget is around £300-500

I know what parts I need to get which mainly include:
Motherboard
Processor
RAM
Graphics Card
Sound Card?
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Some of the things I am not too sure on are:
Power Supply (As Some components require more power, which I don't know)
Sound Card (I don't know which is a good one or whether this is needed)
Hard drive (I May need one which can do tasks better as games today are large in file sizes and a faster speed would be handy)
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I don't want to spend Thousands on this but if you can help me build one which can run most games like MW2 on good performance I will be greatful.

I have a Mac.
Pity me.

Macs are the worst...

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Have a look at some prices... you also need monitor, cooling system + casing
any games comnig out from now will definately need a Quad processor... so get one of them
ask X about graphics n shit...
Cooling system ahve a look on here...

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