if i understand your question right then yes, moving your wow folders off of the HDD that hosts the OS could improve performance. if you are thinking about getting another HDD i would highly recommend an SSD. for multi-boxing all we care about is access times. like Sajuuk said you only need a cheap 30G or so 1 that can be had for around $70-90. wow only takes up about 15G of space so 30G is more then enough. you will have to symlink your wow folders but you should do that already even w/o another HDD.

BUT. and this is a big 1. from the specs you listed above. your best upgrade would be 4G more RAM. if your motherboard can support 8G of RAM and you have a 64-bit OS then go with 4G more ram before anything else.

WoW is very RAM intensive. 8G is just about a must for wotlk.

hope this helps some.

~YYF