Hard Drive help

I have a new MPB 13" 2.4 Ghz 4gb ram and i just upgraded from the stock 250 hitachi drive to a WD6400BPVT (5400rpm). I really want a 640 gb drive (something over a 500gb drive) but this drive is noticeably slower than the stock, its frustrating using IMovie (tons of lag). Anyone have experience with this drive HM640JJ. Good reviews on newegg...will this be faster than the stock drive to where I will be able to use Imovie w/out lag (i'm assuming it would be)...and if not, I might be willing to forgo the 640gb and get a 7200 rpm 500gb drive...recommendations? Basically i just want a drive that i can use Imovie, Lightroom, Photoshop CS3 and maybe even have them open at the same time w/out lag (is that realistic - i hope it is).

Thanks and any recommendations would be appreciated...i'm trying to do my research before i go make a purchase...

check out the solid state drives, a lot faster, but not as much space

Also, i'm on a students budget...trying to stay in the $60-90...SSD and even the xt momentus are probably out of the picture

Seagate Momentus
Samsung Spinpoint
WD Blue