off to buy some bits tomorrow, finally decided to upgrade to PCIE..so close to decision, help me make my mind up please 
currently running a XP2500@3200 and 6800GT AGP
GEIL 1gig DDR ram, so will reuse this ram. I have looked at CPU benchmarks for gaming (my use) and there is not much in it between CPUs, where as the GPU makes a big difference.
Thinking of:
Asus A8N-SLi SE nForce4 SLi (Socket 939) PCI-Express Motherboard
AMD Athlon 64 3200+ Venice 90nm (Socket 939) - OEM (CP-131-AM)
Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940)
OcUK GeForce 7900 GT 256MB GDDR3 TV-Out/Dual DVI (PCI-Express) - Retail
OR
Sapphire ATI Radeon X1800 XT 256MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-121-SP)
regarding the GPU, i was all for the ATI, but not like the threads i have read on the problems people have had, which seem high in s short space of time. Therefore, the 7900GT is pulling me in.
Overall, this upgrade seems to be the best i can come up with for the performance/pound.
cheers, advise away

currently running a XP2500@3200 and 6800GT AGP

GEIL 1gig DDR ram, so will reuse this ram. I have looked at CPU benchmarks for gaming (my use) and there is not much in it between CPUs, where as the GPU makes a big difference.
Thinking of:
Asus A8N-SLi SE nForce4 SLi (Socket 939) PCI-Express Motherboard
AMD Athlon 64 3200+ Venice 90nm (Socket 939) - OEM (CP-131-AM)
Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940)
OcUK GeForce 7900 GT 256MB GDDR3 TV-Out/Dual DVI (PCI-Express) - Retail
OR
Sapphire ATI Radeon X1800 XT 256MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-121-SP)
regarding the GPU, i was all for the ATI, but not like the threads i have read on the problems people have had, which seem high in s short space of time. Therefore, the 7900GT is pulling me in.
Overall, this upgrade seems to be the best i can come up with for the performance/pound.
cheers, advise away