The Basics
Memory, or RAM, is the spot your computer stores and manipulates information while programs are running. Data can also be stored on the system's hard drive, which is far larger much slower to get to. Typically, the more memory your computer has the faster it can run since it's not always waiting on the hard drive.

SDRAM -- A now-fading memory standard capable of working at up to 133MHz. Now actually becoming more expensive because of falling demand.

DDR SDRAM -- An update to conventional SDRAM capable of performing two transfers for every one that SDRAM did. You'll see clock speeds up to 400MHz. Perhaps the best performance/vaue currently on the market.

RDRAM -- A new technology based on super-high clock speeds. Basically, it moved smaller chunks of data more often. Extremely expensive since the devleopers aren't afraid of a little gouging with their patented technology.

Dual Channel -- Dual channel is a way of accessing memory rather than a type of memory. Basically, two sticks of memory are installed side by side and accessed simultaneously. Theoretically, transfer speeds can double. In practice, we have personally seen gains of around 30%, which is actually really good considering that motherboards supporting dual channel access really aren't much more expensive than older ones.

Our Recommendations
First off, based on price and performance we feel that DDR memory is currently the only way to go. It's cheap, it's fast, and it's being supported by all the major manufacturers so you'll be able to get it in the future. Also based on price and performance, we recommend you get a motherboard capable of Dual Channel memory access... it's like free speed. You should also consider buying the fastest memory available (DDR400)... market prices are really close between even the fastest and slower versions of the RAM. A little extra expense now may save you a bunch when it's time to upgrade.

As far as amount of memory goes, here are a few guidelines.

256MB	With the size of operating systems and applications growing daily, this is not really a lot of memory. It's just enough to get Windows and some applications up and kicking but doesn't allow the system to really get efficient. For the price of memory these days, we'd recommend going with more.
512MB	This is about ideal for the "average user". It provides lots of play room for Windows, Internet use, audio playback, and office applications like word processors and spread sheets. This is a reasonable amount of memory, even for notorious resource hogs like games and video editting.
1024MB (1Gig)	This is lots of memory... more than the average user needs. You should be considering 1 Gig of memory if you're into the latest games with everything turned on. It's also critical if you're going to be doing any pro or semi-pro level video editting or 3D modelling.
More than 1Gig	This is pretty much too much memory. Unless you're into intricate video production with lots of cuts and effects, you'll probably never use more than a Gig.