drenger - as you can see in my sig i run a scsi hdd as the system drive and along with it the adobe suite, ms office a few video encoding apps and bf1942 w/ some mods. i feel this is the optimum setup and i still have 21GB free. the system is snappy and runs very well. if for some reason during a game other than map changes the computer hits the hdd, you will not even notice it. just for sh!ts and giggles i took out 1 of my 512 sticks of ram, thus running single channel 1x512MB and with the scsi it still worked well, even with bf and the mods which are known to take ~700+MB of ram. there was just no stutter when it would hit the hdd during gameplay. i then ghosted the 36GB to the 60GB and put it as the system drive with 512MB and the game was close to unplayable, you could tell whenever it hit the hdd. so i would say yes, install the os and game on the faster, scsi hdd.
tostada - i too was wondering and now have my main pagefile on a different hdd and don't really notice any difference, but i rarely if ever go over 1GB of ram so i doubted i would see any difference.
last night i was playing silent heroes @ 1024x768, highest quality w/ 4AA and the game was running smooth like normal, probably atleast 70fps, then i got a couple of stutters, which turned out to be acronis drive image creating the nightly image of my system drive. after the initial hiccup, which most non-enthusiasts would not have even noticed, i was able to continue playing without so much as another hiccup while the system drive was being imaged/cloned to a image on my main server via 100Mb/s switch and then to the other 2 hdds in my system, not all at the same time but one after the other. map changes were barely effected, and gameplay was fine. with that being said, again i would recommend you putting your os and other programs on the faster scsi hdd.