I won't cheat you: you are NOT in black hole

altough you have no chance to put C2D in there.
Your motherboard doesn't support CONROE and successors, which means C2D architecture. But you still are able to put some 2-core Pentium-based CPU, not CONROE Pentium Dual Core, but Presler-based Pentium D 960 which is 32 pounds on Ebay.
This is described here:
http://en.community.dell.com/what-do-i-buy/for_enterprise/f/4516/t/18220539.aspx
As you can see, you may gain some improvement, but under one condition: your actual CPU must be single core Pentium D (not 925/930)

Those Pentium D CPUs are 64-bit, so you can install Windows 7 x64 on your quickie SSD and use DX10/DX11 features in case of better GPU(s). Win7 Home Premium x64 is for 42 quids, if you can search (genuine).
You may try to buy something like
http://www.overclockers.co.uk/showproduct.php?prodid=GX-184-MS&groupid=701&catid=56&subcat=1699
(83 pounds) or used GTX 460 - which normally performs better, especially on BFs, but due to some strange nature of higher dual-core CPU usage by nV drivers with weaker CPUs it may take any role - so this Radeon would be better.
In total you pay 157 pounds, you receive brand new card (possible to sell, if efficiency of such system wouldn't be good enough), brand new OS (may stay for longer, if you decide to buy something else - like platfom change) and CPU worth 32 quids - one and only real, potential, risk
BTW, don't go this way

and don't compare PentiumD+7770 vs C2D+7770 or AthlonII+7770, because it's obvious the latter is more efficient, so don't bother with bottlenecking. Look for efficiency gain.