This Stackoverflow posting has a rundown on workstation hardware. Media production types are a major user of such hardware. You may find something of interest there.
If you're doing anything with video, make sure your disk subsystem is fast enough. Software RAID on Windows in general and XP in particular is pretty rubbish. You might want to benchmark a H/W SATA RAID controller. Some will do stripe sizes up to 1024K, so you could get pretty good streaming performance from a RAID-0 of (say) 4 inexpensive SATA drives. A basic 4 or 8 drive controller from Areca, 3Ware, LSI or Adaptec is just a few hundred dollars. The whole disk subsystem would be just a fraction of the cost of Adobe Creative Suite.
The more memory the better. Single-socket PC's tend to max out at 4GB or sometimes 8GB
of RAM. Two-socket machines tend to be more expensive. Look into something with intel 'Nelahem' processors like a current-gen Mac Pro or HP (Z400, Z600 or Z800). These processors are (a) very fast and (b) can take quite a lot of memory, even on single socket models. Check out pricing on third-party memory as the manufacturers of this kit will charge a fairly steep price for bits. 16GB or more is quite feasible on these machines without breaking the bank.
Note that you will need a 64-bit O/S and 64-bit apps to use this RAM. Hope you don't mind Vista.
If you have GPU support on your software, consider getting something like a NVidia Tesla C1060 to accelerate this. You will need a machine with a suitable motherboard and power supply. Off-the shelf, a HP Z800 or a similar machine would do this if you have the money but they are quite expensive.
If you want to do it on the cheap, there are a range of single-socket workstation and gaming boards available from most motherboard manufacturers that a custom white-box outfit could put into a machine. Also, older workstation models like a XW8600 or XW9400 can be bought off ebay fairly cheaply. They are not quite as fast as the newer Nelahem boxes but the big ones have two pcie-x16 slots and beefy power supplies that can support a high-spec video card and a second Tesla card. They also have the advantage over generic white-box kit that somebody has put some thought into the airflow in the case and the machine is rated by the manufacturer to support this type of configuration.
Finally, consider getting a calibrated monitor like these ones from Barco, HP or Eizo. If you're doing work for production video or print you can tune these monitors to replicate the characteristics of your target medium. This type of monitor can pay for itesl