First I disagree with NOT having FS9 and Windows on the same drive FS9 needs to access Windows files and vice versa, by having the two seperated you are not only losing performance in the read/write area but shortening the life of your hard drives and making your PC work twice as hard. all in which leads to loss in overall performance.
Also more than 1 gig of ram and you have wasted money, Windows can't deal with more than 1 gig, and FS9 doesn't know what to do with more than 750mb of RAM, so by having more than 1 gig is again hurting performance.
I will try and be diplomatic and presume that your posting this in good faith, but this is pure disinformation designed to trip up the unwarey.
To explore why every single part of this post is incorect we must first look at how hard disks work. Basicly a hard disk is several magnetic platers that spin around at high speed and a read/write head that moves over the platters. Data is stored magneticly on the hard disks platers. When lines of magnetic force are broken by the read/write head a voltage is induced. If no voltage is induced then the area of the hard disk the head is moving over contains 0 and if the head moves over an area that induces a voltage then then it reads 1. This may be irelevant but you don't seem to undestand how a hard disk works.
heres the wikipedia entry to check me up: http://en.wikipedia.org/wiki/Hard_disk
having FS and windows on seperate HD's increases performance significantly (asuming that both HD's are of simlar speed and not fragmented). Windows is one of the first things you install on a computer your going to install flight simulator on (asuming your not a wizz with wine) and so gets placed in the area of your hard drive which gets the fastest access. When your windows pc is running system files are constantly read from this area. Flight simulator will be stored in a different and slightly slower area of the hard disk if installed on the same hard drive. A hard disk is fastest when performing sequential red/writes (areas on the disk right next to each other) because fs and windows are on different areas of the hard disks platters the hard drive is unable to perform sequential read writes as it constantly has to juggle between system files and flight simulator files. This type of read right activity is called random access and is much slower than sequential access because the read/write head has to move a long distance over the platters instead of only a short distance.
If you have windows on one drive and fs on the other then the both drives can be performing read/writes that are sequential in nature instead of random access all over the platters. I won't post any performance numbers as this is logical and easily verifiable by a quick google serch.
as for making your pc work twice as hard? well you could get a hard disk in the 1980's how fast were computers then? a few mhz so it dosen't exactly take alot of cpu power to access a HD.
making your pc wear out faster? well as the head of your os hard disk isn't jigging around asmuch then I would say each individual drive is going to last longer than the single drive and if one fails theres a 50/50 chance it's going to be your non critical FS drive

1gig of ram is a great way to spend money and 2gb is even better

Can people stop spreading crap?
*my freind dave informs me the differance "is like 2%" between on hd and a raid0 setup and not worth it. As he has experiance with raid setups I would go with his opinion. As the benchmark scores I based my opinion on were probobly exagerated.