As Lief says, "One thing I will stress is do not spend money on a games machine, as you will be paying for expensive graphics performance that is not needed for PhotoShop etc."
Well, I made this mistake and bought a Dell XPS 600 machine which was a top-of-the-range gaming machine at the time. I also paid extra for adding on to the Dell spec a whazzo nVidia card thinking that would be just the thing for photoshop, etc. It certainly wasn't. Not least because it (its software) wouldn't work with the dual-screen configuration of 2 x ViewSonic VP730 monitors that I use.
So, after a lot of research, I came across Matrox:
http://www.matrox.com/mga/workstatio...ducts/home.cfm. These dudes were the first people (AFAIAA) who designed multi-headed cards. I chose the Matrox Millenium P650 PCIe 128 and what a fantastic card it is. No more Windows blue screens of death and it comes with excellent calibration software. Although Matrox cards don't seem to ship with Dell products (and I do recommend Dell), make sure the Dell motherboard supports PCI Express (PCIe) and then buy a cheap card as part of the Dell spec, then switch it out and replace it with a Matrox.
I use 2 x 300 GB Maxtor DiamondMax SATA hard drives - one as the primary, the other has a clone of my system (Windows and software) and also backup of all my data and photographs. I don't use RAID 0 (which would do this automatically) because if there is a problem with the hard drive you are "copying" from, it copies the errors onto the backup hard drive.
I also have 4 Gb of RAM, but really only 2 Gb is needed.
One of the things is to change your pagefile so that it is the right size for your memory - the default is often not. You can get quite a leap in performance if you do this.
As Tannin said, "Don't ever, ever, ever buy a supermarket vomit box (Compaq, Acer, Dell, Hewlett-Packard, etc.). These things are guaranteed to deliver less performance, with a shorter warranty, and a much higher spare parts cost than anything else on the market. If you want a can of baked beans, go to a supermarket. If you want a camera, go to a camera shop. If you want a computer, go to a computer shop"
That is excellent advice - I work in IT and have to spend quite a bit of spare time fixing family/friends PCs because they thought they'd save money by buying a cheaper PC "supermarket" style. I guess that is ok for them because they have a mug like me to fix it for them when (not if) it goes wrong!
Also, definitely go for XP Pro not home and make sure you have good security (anti-virus, personal firewall, etc).
Of course, the best thing to do is to do a self-build. One of my machines I built myself and it is the most stable machine I have! However, researching which components, buying them and then building the machine is rather time-consuming, hence why I bought my second machine - the Dell XPS. Now I regret not having spent time to build another box because the Dell worked out far more expensive than if I had just built it myself because of switching out the hard drive for 2 x 300 Gb, buying the Matrox video card and extra memory! Next time I will self-build again - it's the only way to get exactly what you want.
Definitely you want a tall box as Leif said - you will be able to fit lots of stuff in there should you need to upgrade in the future. That is probably the only good thing about my Dell XPS apart from the fact it has a groovy light on the front <g>