The problem is the costings price/performance.
One of the fastest core 2 duos is the E8400 £126.14
http://www.ebuyer.com/product/173968
One of the fastest core 2 quads is the Q9550 which is £200
http://www.ebuyer.com/product/152394
For cheaper you can have an AMD alternative that
Athlon II 630 is £84.53
http://www.ebuyer.com/product/176579
ASUS M4A77TD AMD77 is £63.38
http://www.ebuyer.com/product/181964
Crucial 4GB (2x2GB) DDR3 1333MHz/PC3-10600 is £69.99
http://www.ebuyer.com/product/166996
That is approximately £217.90 and you'll get a new socket with upcoming processors coming, 4GB DDR3 memory and similar processing performance. That is £17.90 more than JUST the Q9550 processor alone. - You can then sell your current proccessor/motherboard/ram on ebay for about £90 and recoup.
Now if you are on a complete budget you'll like this!
Athlon II 630 is £84.53
http://www.ebuyer.com/product/176579
ASUS M3N78-VM GeForce 8200 Socket AM2+ is £46.83
http://www.ebuyer.com/product/152752
Kingston 2gb DdD2 1066mhz/pc2-8500 Hyperx Memory Cl5 is £40.97
http://www.ebuyer.com/product/165306
total: £172.33 - its a AM2+ motherboard so its not as new as AM3 motherboards, but AM2+ is still getting new processor support such as the upcoming hexacore processor next year, you are also getting an additional 2 GBs of ram and performance. all that is about £46.19 more than a single E8400 processor and the Athlon II 630 would be faster at 90% of tasks. Also this build is about £17
cheaper than a single Q9950 drop in, granted the Athlon II 630 is only slightly slower than the Q9550 but you benefit from getting more memory (2GBs), a better upgrade path with AM2+ whilst saving a bit of money.
So its your choice, you can go for a drop in Core 2 CPU, but you can practically get an entire AMD build for about the same price which similar performance.