honestly, it is all in how you keep your Hard Drive clean, registery clean and limit the amount of stored data / files / films / movies / etc add infinium.......
as for an external HD....if you wanted to, you could load software on it, you could make it a seperate bootable HD with different OS ( if the HD is a firewire or a eSATA drive type....USB I think would be a tad slow response/access timewise )
as for differences in speeds.them raptor 10,000 rpm drives ROCK...the fujitsu 15,000 rpm drive ROCKS as well......just cost more.....
I use currently (2) hitachi SATA 160 gig drives in RAID 1 setup..and don't see any performance hit even though my delapidated athlon XP2800 ( at stock clock ) and 1 gig ram is good a worn out..... Antec TP-II PSU and ATI X800 series 256 meg vid AGP is making /writing everything twice at the same time.......1280x1024 screenres 1024 textures, and most times maxed out on detail sliders and stay steady 70 FPS...unless scenarios or FSO at times.I dip in 50s or so.but if I lower sliders I am right back to max refresh of 70......
sorry got off track a wee bit.......
so if you want to game..just load up OS< minimal extras, good clean registry, ( I set my page file to 4,000 ) instead of letting windows manage it for me......
and the smaller HD the better 20 gig is good....80 gig is pushing it...... a raptor 36 gig or 74 gig is ideal.... make a dual boot system or load up winXP and AcesHigh2 and the needed Aces High textures/plane cache/terrains.and nothing more.....
then have other HD for all other PC use......
just my view of it, everyone has their own opinions/views...........figured I give mine.......
just be sure to backup your data/important files often, and keep your PC clean, registry clean, defragged and you will be fine........