A full HD monitor is a must and check reviews for colour quality, you need one that is definitely accurate and represents colours well. Also don't be too worried about spending money on a very up-to-date graphics card, unless your system will also double up as a gaming machine! Obviously you will be needing a large hard drive, though you may wish to consider either Western Digital Velociraptors and RAID setup for even better performance. A Solid State Drive (SSD) would be even faster, but these are still very expensive for their size and so may be worth waiting for prices to come down a bit.
Otherwise I agree, 4 - 8Gb of RAM, 64bit OS (else not all the RAM will be recognised) and one of the latest quad core CPUs to keep you up to speed for a long time.
Hope that helps