I'm still living in the land of the Northwood, and therefore use DDR1. However, as far as I'm concerned, any heatspreader is good for use in DDR2. They're just 2 pieces of aluminium, with the RAM wedged in between the 2 pieces, with a clip holding them together. No screwing, no catches. Worst case scenario is that its too small for it, but having the heatspreader will still be a plus till DDR2 sizes come out.
After taking a look at a couple of pics, the size seems the same as DDR1, so I wouldn't be worried about that.
About your second question, I don't remember anything said about running SLI utilizing more RAM. I won't be concerned about it.
For current applications, 1gb seems like a good place to be in, although some people might be more comfortable with 2gb of RAM. These people usually include those that do CPU intensive stuff such as video editing, CAD, etc. Most people don't, and the performance difference is quite minimal to bother with if you don't use those applications much.
I currently have 1GB in mine, although I have the option of running 2GB (I've got the RAM in my spare comps). I'll run a test this weekend to see if 2gb would actually make any difference. I doubt it though.
In any case, this is my recommendation: You can always increase the amount of RAM in your computer when you feel the need for it. Leave it as it is, and if you feel your computer being too slow in the future, add more in. By that time, the RAM in the market should be faster and cheaper.