If you are saying which should I buy for an EXISTING motherboard, it will be determined by which slot you have in that board. In other words, your existing motherboard will have either an AGP or a PCI-E slot, but NOT both.
If you're asking which is better, then PCI-E is the newer technology and has faster data transfer rates than AGP.
I amj just thinking of it as a hobby I would be in no rush to get it built and I would like to build a super gaming pc maybe over the course of a year cost would not really be a problem as it would be something that i would enjoy and have something really good at the end
You'd be best just saving up the money then buying all the components in one go, otherwise you may buy something at the beginning of the year and when you're buying your last bits you'll find that the bits you bought at the beginning of the year are out of date.
To give you a kind of price guide my current PC cost $4000 (about 2000 pounds), but I had to buy everything barring a 300gb and 200gb hard drives.
Once you have the money sorted, post back and I'm sure people will give you advice on what parts to get. Have a trawl through overclockers.co.uk and have a look at the goodies there to give you an idea of the prices.
C3 is right I'm afraid. As an example my graphics card was £130 and now the exact same card is about £60. Admitadly thats a major example but it gives you some idea. In many ways the longer you wait the better but you have to start sometime. I always pick the wrong time, I built my socket 939 based computer just before AMD announced their new socket AM2. Wouldn't mind so much but the AM2 chips are cheaper than the 939 equivalents. Thats the way it goes I suppose.