I am looking into buying either an Intel Q9300 or Intel Q9450 processor.
I will be running audio applications with high resolution files and DSP hungry  
plug-ins. Besides the slightly different clock speeds of these two chips, the  
Q9450 has twice the amount of L2 cache (12MB) than the Q9300 (6MB).
 
Does this L2 cache have any effect on how many plug-ins I can run at the same
time and how fast plug-in bounce times are on the file?

really the cache wont affect the plug in directly thing like that just go to add in to make a better cpu, it not like whole apps are stored their.
 
the q950 is always going to be faster, bu that not jsut because of the cache but the increases clock speed it.
 
these are very good chips and i can only reccommend them. they will both do what your after but, the q9450 will be faster but buy what you can afford.
 
PS: if your running vista for audio work get a x86-32 version as not all audio h/w has mature enough to run in x64 for the time being

Thanks for the info. Not sure if I'm going to run XP or Vista. I wanted to try XP64, but my apps might not support it.

yeah very few audio app will run well on XP64 and even less hardware.
 
vista isnt at all bad