You have a few options.
Your ram should be fine for what you're doing. Keep your system running as lean as you can so that you're not wasting CPU power on stuff you don't need to have running, like 'quicktime news update'.
Do the obvious by defragging your sytem and cleaning it up.
To speed up your clips:
1) use the bait and switch method. Blazingly fast, and smooth to work with, but a pain in the ass to keep track of.
1.2) use another codec. Something not lossless that runs more smoothly but still looks good.
this one was done in Mjpeg as an example
2) Get a really fast hard drive - and keep it dedicated for editing. If you have an older system you might want to consider getting an SATA controller card instead of using IDE.
3) Are you editing in WMM, or premiere? If it's premiere, try rendering your clips out, and check what codec your preview renders are in.
4) annoying but theoretical speed boosts - Check your motherboards frontside buss and compare it to your ram. If you have an 800mhz fsb and 300mhz ram you can get better stuff.
4.1) overclocking - At the least, you can look into high quality cooling. If your computer is running hot, it'll er... 'de-cycle' so to speak and slow down.