The 1333mhz FSB Core 2 Duos are cheaper than their 1066mhz counterparts because they are based upon a more 'mature' process, thus being cheaper to produce. If you are buying a Core 2 Duo there is no point in buying a 1066mhz model as they are more expensive, and performance is very slightly less. If you want a Core 2 Quad though, they only come in a 1066mhz variant (with the exception of the horrendously expensive QX6850).
Indeed, you do want to make sure you get a p35 based chipset as they will offer some degree of futureproofing over the 965 and elderly 975X chipsets. If you have a lot of money to splash about, you consider the X38, but that has features that are unecessary for the average user). Here is a selection of P35s click here Performance for all of them will be very very similar, it just depends upon your price limit and the features you are looking for.
If you get a processor with a 1333mhz bus get 667mhz (PC5300 RAM). If you get a 1066mhz bus, 533mhz is better. This means that the ratio between the processor and ram match, which can often be quicker overall than having faster ram. eg a 1333mhz bus is equal to 10,600mb/s which pairs nicely with dual channel 667mhz (2x 5300mb/s) ram. You only really need faster ram if you want to overclock.
For a graphics card around the £100 mark I would suggest a Nvidia 8600GT or Radeon 2600XT (GGDR4). These are both DirectX 10 which will also allow for some future proofing. If you will be running XP though, and dont care much for DX10, you could get an older more powerful card such a Radeon X1950Pro or XT for a similar price.