from what I understand you have 1gig in 2x512MB at hand and are waiting for your 2x1GB modules. It truly doesnt matter if you use the computer with one gig of ram and then just upgrade when your new ram gets there, you can just swap it out without problems. The only time you could have problems is if you overclocked with the 1gig and then just tried to plug in the new ram.
So basically, it depends on whether you want to start setting up the computer right now or wait for a few days.