Performance wise I would go for the RS4, but TBH I don't like the looks of the RS4. The looks are not sporty or aggressive enough. Porsche - as someone said you won't get much for 100K - a second hand Cayman (not the S either) maybe which outs out similar figures to the standard WRX. What about the new 1 series BMW that comes out in 2008. Top of the range 1 series comes with a 3.2l V6 twin turbo (same engine as M3) and the price that has been suggested is about $75k (I think). Performance wise you're looking at 0 - 100 in 5.3 sec which would kill a Porsche Cayman..

Someone mentioned a Lotus, I considered one of these and you get fantastic car for the track, but you it wouldn't want to be your daily drive.