Quixote said:The good news is your CPU is pretty good.
The first thing to do is replace the graphics card - this one
http://www.overclockers.co.uk/showproduct.php?prodid=GX-042-OK
for £115 will perform great.
Your hard drive is recent & probably pretty decent. As mentioned, it's probably that you could do with more ram.
You will almost certainly have DDR2 Ram with your Pentium D. Look how many slots your current RAm is using & how many you have free - is it 2 x 512mb or 1 x 1gb. Do you have four slots in total or two.
If you have some free slots, I would add another 1gb cheap ddr2 ram, such as this for £85:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-083-CS
With these parts, you should not have to worry about changing your PSU as the 7900GS has quite low power requirements whilst still being quick.
I've made these recommendations with a view to getting a big performance boost without spending silly money.
I would recommend getting the free program cpu-z to find out what speed your ram is so you can buy matching. Also, confirm if you have free slots for adding Ram & double check that you have a PCI-E slot for the graphics card.
This will make your PC pretty damn good & should run most games includeing BF2 at pretty high details without a hitch.
intel pentium d 930 3GHZ
motherboard
asus lithium 1.04 with 4 slots
chipset
intel i945P
Ram is 2 sticks of DDR 2 512 (infineon)
Graphic interface: It supports x16 (does this mean I can get a BFG 8800??? rofl!)