I buy from ebuyer.com, but I was told Scan.co.uk is better (for UK). If you are making a new build I'd definately look at getting a dual core CPU, it will handle a lot more now, and prove itself in the future. If you are going to be running XP Pro get the 64bit if your going to keep it, or maybe pay less and get the 32bit, waiting for vista.
If you are looking to getting a good graphics card, and you have quite a lot to spend on it, those nvidia dual gpus are good. You could also wait until 2006 Q4 for a DX10 card, which will perform best on vista, and better in games. Although they will be expensive to start off with.
Id look at getting a 64bit AMD X2 - they are so good for the money. You should get the socket AM2 - from what I was told. The socket 939 would be fine for a long time though. The way you know the things are compatible it cause it will say in the title.
Motherboard: ASUS A8N32 SLi Deluxe NF4 SLI, S939, PCI-E (x16), DDR 400, SATA II, SATA/IDE RAID, ATX.
This motherboard is "ASUS A8N32". It is capable of SLi (SLi Deluxe NF4). Its socket 939 (S939). The board takes PCI-E x16 cards. It takes DDR 400 memory, and SATA II and SATA HDDs, as well as IDE.
Its pretty straight forward, just match things up, they make it easy on these sites now. If you need any more help post back here, and I will get back to you, or maybe even someone else.