If tempted to spec up your own machine I advise caution. In this hugely competitive market, the big boys are working on margins sharper than the edge of a razor blade. Independants have no way of being competitive with this lot. An example of "How to blow a budget" will include this. A nicely hidden cost is simply that of Windows XP which in OEM form will set you back £130 for Xp Pro or about £90 for XP unPro. Nicest thing is that Microsoft will not and do not support OEM installations, so you get customer support of absolutely zilch for your money.
Depending on how long you want your machine to stay current, many of the boxes on sale now will do photo and video editing quite well. I do a lot of photo editing etc. with Photoshop and your main requirements if like me, you are serious about your photography will be something like this. Lots of RAM - and not less than 1024mb. Lots of storage. - I currently run a total of 520GB and am filling it fast. (each photo about 6MB) Processor and peripherals - Not as important as you might think. Thirdly, do mke sure that you have good Firewire support. In this digital World, it is unlikely to now find a machine without it, but if you do, you are going to be stuffed. Multi media isn't a top end requirement nowadays and a good mid range machine will be viable for some time to come.
Yes, I specced up the machine on which I type now and it is a really good performer. Sadly, through building and upping the spec. it cost me slightly more than two new Typhoons. From now on, I will buy ready made as the general level of performance has improved so much in recent years. I would never - never, never however, go to P* ****d.