you can upgrade with a new graphics card steve but its gonna have to be a pci interface card
this limits your options,however there are a few good cards avalible including one which has pixel and shader model 3.0
the PNY GeForce 6200 256MB PCI DVI TVO (http://www.dabs.com/productview.aspx?Quicklinx=4CQQ&CategorySelectedId=11137&PageMode=1&NavigationKey=11137,41600000,353180000)is a excellent pci based card.with pixel and shader model 3.0,and core clock of 300mhz and memory clock of 550mhz,these are excellent speeds for a pci-bus interface card
you wont get lightning speeds from this card,but you will get frame rates around 40 to 50 f.p.s(frames per second),which is good
this card is normally cooled by a heatsink so make sure you have adequate cooling in your case
another good card is the NVIDIA GeForce FX5200 256MB (http://www.dabs.com/productview.aspx?Quicklinx=45BW&CategorySelectedId=11137&NavigationKey=11137,41600000,375830000),this has pixel and shader model 2.0 and is vista ready,its core and memory speeds are 250/333mhz respectively and has 256mb of video memory
this is a good all round card,especially good for direct x 8.1 games such as "castle wolfenstein" and "hitman",and also "max payne"
you can also run "doom3" and "qauke4" with this card,however your processor speed will also be a factor when trying some games
other cards include the Sapphire Technology Radeon 9250 128MB(http://www.dabs.com/ProductView.aspx?Quicklinx=3M6K&CategorySelectedId=11137&NavigationKey=11137,41600000,43280000&InMerch=1)and the Sapphire Technology Radeon 7000 64MB (http://www.dabs.com/ProductView.aspx?Quicklinx=2MPZ&CategorySelectedId=11137&NavigationKey=11137,41600000,43280000&InMerch=1),both these cards are inexpensive ,but their overall graphics performance is average,but if your only playing old 3d games and the odd online game these cards are just the ticket
also you may want to take a look at the matrox selection of pci cards such as the Matrox Millenium G450(http://www.dabs.com/ProductView.aspx?Quicklinx=3JT6&CategorySelectedId=11137&NavigationKey=11137,41600000,43970000&InMerch=1)and the Matrox Millennium G550 (http://www.dabs.com/ProductView.aspx?Quicklinx=3VSD&CategorySelectedId=11137&NavigationKey=11137,41600000,43970000&InMerch=1),these cards are not gaming cards but more c.a.d(computer aided design),and the later card is a low profile model,which basically means it will fit in cases were space is a issue
these cards may seem a bit expensive but if design and c.a.d is you thing ,these are good solutions for your p.c
finally steve,just for a laugh,why not splash out a £1000 on a Matrox Parhelia HR256 256MB(http://www.dabs.com/productview.aspx?Quicklinx=3Y6Y&CategorySelectedId=11137&NavigationKey=11137,41600000,43970000),this is a top of the range design card(in pci-bus interface) and is excellent at producing crystal clear visuals,and these are actually used in hospitals in c.a.t scan machines to produce 3d x-rays of the brain,so you know these have to be good
personally steve,i would go for the geforce 6200,a good card and probally the best all round pci-card you can buy
any probs mate let me know
good luck steve!