the q9550 is a quality processor and so is the e8600... they both pale in comparison to the i7, but if you dont wanna spend all the cash on DDR3 and an x58 board, they're great picks... it really depends what you wanna use it for... the 8600 will achieve higher clock speeds because if the higher multiplier and, being a dual core, ability to achieve higher FSBs with stability... this high clock speed is great for single and dual threaded apps and games (which is still almost everything)... the q9550 is better for heavy multitasking, video editting, complex mathematics, encryption, compression and other multi threaded things... the q9550 will be more future proof, as nehalem will bring forth many more multi threaded apps and games
speed is limited by cache, but only at an asymptote value determined by cache bandwidth as a function of clock speed... so in short, not at normal speeds.