My old dual core motherboard couldn’t provide the necessary voltages to run a quad core, it can be pretty tricky.
I made my system last as long as possible (a bit like Trigger’s brush, probably one system in 10+ years and lots of upgrades until nothing of the original remained) but finally went the whole hog and upgraded from a dual core Pentium-D with AGP graphics card and 2Gb ram to a quad core i5 with 4Gb just before Christmas and the difference is amazing.
There’s only so far you can push a given system so if you can afford it I’d say that a new base unit with Windows 7 would be the way to go and then upgrade that as and when in the years to come until you need a new unit again. You can get some suprising cheap quad core systems these days but there are also so good deals on cpu/motherboard/memory packages but then you’d need to add in a gfx card, operating system and possibly a new psu.
If you get offered 32bit or 64bit Windows go for 64bit, I made the mistake of going 32bit without realising that there was a 4Gb memory limit for 32bit Windows!