It won't be process-the-whole-of-the-internet-in-twenty-seconds kind of improvement, but yes, lower latency RAM will provide an at least somewhat noticeable gain in most cases.
It depends on what you're trying to do. If your CPU and hard drive are fast, you can decide not to preload things to free up RAM for calculations.
If your CPU is busy enough with flight dynamics or you've got an IDE generic hard drive, load up on mid- to high-quality, high-volume RAM. Buy matching sticks and make sure they're running in dual-channel mode. Turn on preloading in the game and you'll see quite an increase if the system will handle it.
My gaming computer has 4x512 PC3200 DDR sticks in it in dual channel and a SATA hard drive at 10k RPM, meaning I can preload even the hi-res textures. (When the system runs out of RAM, it uses something called a page file on your hard drive to act like RAM, hence why the hard drive even plays a role). That way when I preload too much, it spills into a drive that's fast enough to at least fake like it's keeping up with the RAM.
