Well i guess i will try and give you some advice!!!
Im guessing that if you deal with sound etc then you will need quite a powerful setup, so first things first is a dual core processor as this will allow you plenty of scope for multitasking. Then you will need at least a gig of ram, then obviously the sound card.
Have you ever heard of Novatech? if not they are a warehouse based computer company and are about the cheapest around now they have Barebone systems, which is what you're looking for (no monitor, keyboard, mouse etc) my recommendation would have to be this offer here click here a core 2 duo processor, 1 gig of ddr2 ram, a decent future proof motherboard and case all for £280 which comes pre built and tested.
Now for your soundcard im not such an expert with sound as maybe you are but ebuyer have some cracking deals. This link will take you to a good soundcard for £50 click here or if thats not good enough then you have this next one for £185 click here but it obviously depends on what you want it for. Hope this has helped a bit, Kev