It's safe to leave it set to System Cache. System Cache will try to load programs that are running into memory and leave them there (even after you exit), so that the program runs more responsive. It is dynamic, meaning that as other programs require the memory XP will automatically shrink the cache so as to avoid memory conflicts. If you have poorly programmed drivers (in my experience SiS drivers cause the most problems) you can run into memory issues in which case you may not be able to use the System Cache. You will most notice the effects of System Cache if you run a program you have already used before, as long as you havent rebooted in the meantime. For example, you played SH3 for a while, exited the game to eat lunch, then went back in, it should load noticeably faster the 2nd time, especially if you have lots of RAM and havent run other programs in the meantime.
A program I use in troubleshooting many computers is Autoruns. It used to be by SysInternals which was bought out by Microsoft. It can display any startup program in pretty much all locations. It can be found here:
http://www.microsoft.com/technet/sys.../autoruns.mspx
Also, here is a good reference for a whole lot of ways to maximise performance in XP.
http://www.tweakxp.com/performance_tweaks.aspx
Note that some of these are controvertial as to whether they really do much to help or not. Tweaking your system is mostly only useful to people with minimal system requirements. For example, when I first loaded Oblivion onto my laptop, my system achieved around 30fps in general. I had a ton of background stuff running (desktop enhancements, widgets, all kinds of junk, no antivirus tho). I then decided to tweak my XP, so I set up a dual-boot, shut off all unneeded services and startup items, applied most of the performance tweaks out there, etc etc and reloaded Oblivion. Guess what....I still only got about 30 fps walking the same test route as i had before in Oblivion using the same graphics settings as before.
So I tried the same thing on a friend's desktop...he could barely play the game on the lowest settings in Oblivion. So I optimized his system as I had mine, and reloaded the game....it turned out that the game ran a bit faster....except in parts where there was heavy graphics usage (ie: water, fog, lots of activity...), where it still slowed to a crawl.
So to make a really long story short.....how much performance you might gain by doing all these tweaks in this thread really depends a lot on your computer vs the requirements of the game. A computer that is min spec...is still going to be min spec no matter how you tweak the software
btw: sorry for the long post but this seems to be a fairly well-read topic with lots of good advice in it
edit for clarification