No, it probably won't matter because I remember hearing somewhere that the data transfer speed is usually the same between the two speeds, it's just a matter of how fast the platters spin and from what I've also heard, 7200rpms wear down faster. :\
The other day I stripped an old P1 machine, it had a WD 814Mb disk (4500RPM?)
Hmm...I was wondering about that because I wanted a disk for mirroring real important data...so I installed it and there is no noticable slowdown at all.
Just make sure your booting and running programs from the faster drive.
Oh...and play with having the swapfile on different drives. It *might* be faster on a seperate drive. (definately if their on different IDE channels)
(or better still: if you've lots of ram to spare (1Gb or so) ditch the swapfile completely!)