the multiplier is partly what determines the speed of the cpu. for instance, e6600 has an effective fsb of 1066 which is actually 266 x 4. the multiplier is 9, so 266 x 9 = 2.4ghz, the overall speed of the cpu. some 'extreme' edition cpus allow you to increase the multiplier over the stock setting, so increasing the speed of the cpu without having to alter the fsb.
as for the 'true quad core' comment, essentially the current intel quads are actually 2 dual cores stuck together, so it's basically 2 x 2 cores, with each set of 2 communicating over the fsb. In theory having all 4 cores on one piece of silicon would increase speeds, however in practice it's mostly just AMD propogandists trying to find any way they can to trump intel, since AMD's first quad core offering will be launcing later this year, and will have all 4 cores on one die.
Anyway, I would definitely go with a quad core, it will be a better long term solution if you want to avoid upgrading again for a while, and the prices are coming to down to almost silly levels now, after july 22nd a Q6600 could be about half as much as an E6700 was 4 months ago