Maybe... maybe not.
First you have to find out how much of the 4GB you can actually use in 32 bit mode (i assume you have 32 bit winXP), which is determined by yout motherboard
Not counting motherboard limit, you can you 3.5Gb in 32 bit XP. I'd imagine with an E6700 processor, you motherboard will take 4Gb or RAM or more.
The performance increase in FSX should be notable, but not as notable as if you were running Vista. I'd say that if you really want to upgrade, go for 1Gb more, making 3GB. Remember though, memory works best in matched pairs!
Also, if you want performance, don't go with cheap memory. For FSX DDR2 @ 800Mhz and above is good. Check your motherboard supports this first though (you may still be on DDR RAM). If this doesn't make sense, you're not ready for a memory upgrade
