4 gigs of memory is more than enough to work with SD video. Of more importance is your processor and amount of hard drive space. Dual core or quad core processor of at least 2.4 ghz with a good, fast graphics card. You will also need a firewire card if you do not already have one, preferably a 3 jack card. They are relatively cheap and plug and play installation.
Video files, even SD video, are extremely large. If you have a single internal hard drive only and it’s big enough, I would suggest you partition it off and keep your programs on one side and your working files on the other. Better yet is to have two internal drives, or external drives for working with your video files. Your system will run much quicker and smoother if you keep your software programs (including your editing program) on separate drive than your working files.
I canned Avid and went to Vegas several years ago. Great program, less expensive than Avid and Adobe, and not power hungry on your system. There are several good programs and you can download most of them for a try before you buy for 30 days free. Pinnicle is the only one I'm personally familiar with that I would recommend avoiding.