Usually, that means placing them in two different slots that are the same color. Placing them right next to each other will not allow them to run in dual channel mode.
If your spending the $$ for a PD 920, don't short change yourself with only 512Mb RAM. 2x256 just doesn't get it and a 1Gb 2x512 isn't that much more theses days. Trust me, when you decide you want to add more RAM you'll be happy you started with the 1Gb. Also, confirm in your manual the actual RAM placement for Dual Channel as usually it's DIMM's 1 & 3, 2 & 4. They tend to be color coordinated. Good luck.
Get the dual channel RAM 2x256 and make sure you put them in the right slots for your board so they run dual channel. If you can, get at least one gig of ram too. Will help performance quite a bit. Usually each motherboard manufacturer has the same colors representing where each ram stick goes, but they can differ.
Get one 512 stick. Get another 512 stick in the near future. Your machine will love you for it. 1gig is the sweet spot right now