Steve,
PC speed is down to all its elements - and also depends on what you are doing. So, obviously if you spend a lot of time move large amounts of data a fast hard drive is good (and as windows is continually reading/writing to hard drive its always a good idea to have fastest available).
So, if you're rich enough, just keep your machine at the cutting edge in all respects.
Most of us can't afford this - so some sort of trade-off (upgrade one or two components, and hope they have best impact). I don't know what your criteria are, what you use most etc (eg, if you were a gamer, fast, high memory graphics card would probably be priorty). I'm not a gamer - and one thing I've noticed since memory prices dropped and I can afford what I think machine needs - faster processors don't make a lot of difference to apps I run. But that's my use. Your original post suggested a decrease in performance since XP installed - but now you just seem interested in performance generally.