definitely get the i3, it is dual core, but also has hyperthreading, so it's effectively a quad core chip, and as others have said, it will eat the q8400 for breakfast.
as regarding to i3 2100 or 2120, you mentioned that the only difference is the multiplier, 31 vs 33. as you may or may not know theses are the so called 'unoverclockable' i3s, as the overclockable 1155 chips either have a k at the end of their model name, or supports turbo. those 2 i3s saddly don't have any of the 2 i just mentioned therefore their multiplier is fixed. i don't own a new line intel 1155 chip, but i've heard that with the multiplier fixed chips, you can only mess about with the base clock which is set to 100 by default, and you can overclock the baseclock to a maximum of about 105. now, if that's true, you can get the i3 2100 to a maximum of about 3255mhz (105x31), which is still a bit slower than the i3 2120 at 3300mhz, not to mention that you can actually overclock the 2120 to 3465Mhz (10x33). so there you go, as far as overclocking and clock speeds go, that's the best i can help you with.
can't find a much detailed review on 2100 vs 2120, but here's a rather simple one
that you can at least get a basic idea on how they compare. bottom line is, the 2120 is obviously better than the 2100, but not by a much, if money is indeed a problem, and you are willing to sacrifice a little bit of performance, then get the 2100.