If the computer I am looking at comes with 256MB of DDR SDRAM, does that mean that it actually has 512MB of RAM because DDR RAM is double-sided?

No

unfortunately, it just means that it works more efficiently.
also, there are many complaints about apple's implication of ddr currently...

Yes. the G4 'pipe' to memory is 133Mhz. They've put faster memory (DDR), but the G4s that are in those machines can't exploit it at all.

This is:
1) Marketting; they needed the 'DDR' and a new box on the market
2) Half bred machine; This is obviously a new chassis and motherboard, but they don't have the proper CPU yet

So, IMO, if you can buy a February 2002 2*1Ghz QuickSilver, buy THAT. It's much quieter, will be much cheaper, and has proven to be solid and reliable.

Is 133 fast?

Also, do you know where I can buy a Quicksilver? Are the Quicksilvers comparable in performance though? Do they use the XServe technology?

If I buy a Quicksilver, that means I have to load and also buy Jag right?

these machines were primarily released to show apple to be using a faster system buss/ram. it's the numbers game again!
ironically (new) pc's use a much faster system buss than this.
it can be argued that the architecture of the apple buss is superior to the "wintel" buss, but why have i heard that apple are seriously considering using intel processors in the near future?
wintel machines, these days, are not quite the inferior "mutant" brothers of apple that everybody still believes them to be.
apple, these days, is REALLY about the superior operating system. this has never been questionable.
on this basis, apple remain the smartest choice for "creative types" like us.
you don't have to use jaguar with any newer mac, and currently, xserve technology definitely supports the "quicksilver" range AND beyond...