256 sticks are now a waste of money for a newer system. The nowadays system needs a gig of ram for good gaming and general usage.
If you got 2 256MB sticks, that would leave you with 2 or less slots of ram left, and that means you would have to buy larger modules to stop swappage, which means waste of money. 1GB chips are way more expensive then 2 512 chips.
Just get a 512 now and a 512 later.