Right so u've got an athlon XP 2600+ so that means it'll b a socket A, which also means u'll have only DDR ram. so to upgrade u really need to get a whole new motherboard, CPU an ram. but wat i gota ask first is how big is yr case, coz usually corporate cases are slightly smaller than normal cases. and if yr doing gaming yr gona need a new graphics (u cant play new games with on-board graphics, it just doesnt work).
so this is wat a think wud be good for u.
http://www.ebuyer.com/UK/product/117990 Foxcon mobo with 5.1 audio on board 2x IDE ports (which is rare) 2x SATA ports (for hard drives) an PCI-Express slot (for graphics cards)
£27.99
U picked a good time to buy ram coz at the moment its never been cheaper!!!!
http://www.ebuyer.com/UK/product/98710 2GB kit of DDR2 800MHz ram. corsair is very good quality ram. it comes with a LIFETIME warranty and i never never had or even heard of ppl having trouble with their ram.
£70.58
http://www.ebuyer.com/UK/product/124915 AMD Athlon 64 X2 5600+ (dual core an a fast 1 at that!!!) Now ppl do have different preferences on Intel/AMD personally i went for AMD coz there really good, cheap and better for gaming. it runs at 2.8GHz and trust me is blisteringly fast!!!! comes with a 3 year warranty
£109.95
http://www.ebuyer.com/UK/product/127241 BFG 8600GTS OC 256mb graphics card. The 8xxx series of nvidia graphics cards are all DX10 capable. if u dont know wat DX10 is basically its the new Direct X from microsoft that is the next generation of computer gaming here's a pic just to show the difference DX10 makes

to run this graphics card u need a minimum of a 375W power supply so u'll have to check that. so u mite need to get a new psu
£129
with next day delivery it all comes to £348.33 Inc VAT which leaves u enough money left over for even a new monitor if u wanted. the above spec wud b very good and wud play all the new games no problem. Ebuyer is very good site to buy stuff from i buy from there all the time purely because the service is great and there cheap.