Hi Stuart R
Why tie yourself down with an expensivea ready built PC that is not designed for the type of work you intend for it, most of the brand name computers such as Dell, Compaq etc, are compromises that work well in an office environment but are lacking in the performance stakes. The only way you'll get the computer spec you would like is to make it yourself, or have someone make it for you. It's certainly not necessary to go for the latest all singing and dancing CPU, 2.8 to 3.6 mhz is more than adequate for graphics manipulation, certainly go for more RAM, but make sure the motherboard and bios support it, some don't like working with large amounts and will in fact slow your computer down. Hard drives are now very large and very cheap, a 300Gb drive will set you back £70. DVD re-writers are very cheap, £24 for a 16X. You don't need to go for the latest PCI + graphics card costing an arm and a leg either, a decent quality card which is more than adequate for 2D graphics and a bit of 3D shouldn't cost you more than £80. Make sure the power supply is up to the job, many are underpowered and will shut down or worse if overstressed, more is better here, 400 - 500 Watts. Assuming you don't intend updating your monitor you should be able to build a very useable PC for £400 or less using quality branded components, you may find some of the components in your old machine can be re-used. I have built many computers for myself and others, quite a few are used for graphics with no problems whatever, none use top of the range and expensive Intel CPU's, all use middle range AMD Athlon XP or 64 CPU's. (no, I'm not touting for business).
Check out PC component prices at the following web address.
http://www.ebuyer.com/customer/home/
nirofo.