DDR2 refers to a type of memory. DDR means Double Data Rate, so it transfers data twice per clock cycle. The Front Side Bus on current Intel based mother boards moves data 4 times per clock cycle. Therefore you should divide your FSB speed in half to choose the appropriate RAM. For example - A motherboard with a 1333Mhz FSB should have a minimum of DDR2 667. You may install faster RAM, but the gains aren't linear.
I see you've asked several questions... Pardon the judgement, and it's hard to say this without sounding harsh, but I'll try. The nature of your inquiries have the appearance you may be better off having a local shop build your system for you.
The 2 in DDR2 is for second generation Double Data Rate. Any DDR module transfers twice per clock cycle. So DDR3 transfers like DDR and DDR2 but it is third generation DDR.
Close, but no cigar. DDR stands for Double Data Rate, as Scotteq wrote. DDR2 means that the ram doubles the data rate of DDR ram, while DDR3 means that it triples the rate of DDR ram, at least theoretically. Things don't quite happen linearly, but that's the general idea if the numbering. Also, because of overclocking, a premium set of DDR2 is currently almost as fast as DDR3, though that will undoubtedly change as DDR3 matures.
Scotteq, you'd be right in saying that - I am going to have someone build my rig for me (no question about it because I am a self confessed newby to computers), but I'm trying to learn by asking lots of questions. I didn't even know what cpu or oc stood for one week ago, but through these forums and websites I now know way more than the avergae non-tecky person and the system my 'builder' recomended me is not nearly as good as the one I've designed so far using other people's feedback. He was trying to sell me a QX6700 - but why when I can get a Q9550 half the price and don't plan on overclocking, he didn't bother to ask whether I would overclock, just wanted to sell the pre-made unit he had so I've come here to learn for myself and then will hand him the design I want, not what he or some other sales guy wants to sell me.
Also I'm honing my questions for answers I need, for example, I want an FSB with a minimum 64 bit capability and preferably 128 bit - does anyone know what mobo I should buy to have these things? I ask this because I've read that wide fsb buswdith allows the ram and cpu to exchange data way quicker etc?
While true, unless Im mistaken (A distinct possibility) anything with less than a 64-bit FSB is going to be from a previous generation of tech, so any modern motherboard should meet your needs. However, I am far from an expert in such things, so dont take this as gospel.