JBT is right - a $90 gig of corsair value select is a great deal. Also, since DDR2 has reached price parity with DDR, I find it likely that production of DDR is being reduced somewhat to make room for the newer technology. This means that DDR will likely not get much cheaper, and may in fact go up in price as supply starts to go down.
As for what would be best, it depends on your system. If you plan to get a socket 939 system, then it might be better to get 2x512 rather than 1x512 or 2x256. A lot of the 939 processors (newcastle, winchester) don't like working with 4 sticks of DDR400, so adding another 2x256 would force you to drop down to DDR333 and 2T command rate. And buying 1 stick of 512 is also bad, since you don't get the benefits of dual channel, which often equate to about 300PR points worth of performance. I guess if you buy a venice core, it will allow you to run 4x256 without too much penalty, but I personally don't like the idea of maxing out my slots with 1GB when the board can support 4GB. Socket 754 is also bad for using lots of modules, but you might be able to get by with 1x512 now and another 1x512 later. But 754 is not a very good platform, since its dead ended, has no more speed grades coming for upgrades, and doesn't have PCIe, SATA-II, etc. The only platform that really plays well with 4 sticks is Intel, but they aren't very good for gaming.
So basically, unless that extra $45 saved can buy you something super, super sweet, and you can't stretch your budget by that amount, then I'd really just get a gig. Performance in games will be a lot better - in HL2, my load times went way up and my games stuttered a lot when i was temorarily using 512MB.