A few Golden rules about Crossfire or SLI:
A single, faster card is better/more cost effective 95% of the time. Don't buy 2 lesser cards like 2600's and think they will outpace a single 2900. The single 2900 will be better. Buy 1 -2900 now, and when you have the cash, add another later. Don't buy 2 lower or midrange cards and immediately throw them in Crossfire or SLI, again the money is better spent initially on a single high end card.
Use 2 of the same identical cards. If you use a slower card in the pair, the faster card will default to the speed of the slowest card.
If you get 2-256meg cards, you will not have 512 meg of usable memory. You will still only have 256 meg of available memory for the frame buffer. If you want to play a game that needs 512 meg of video memory, EACH card must have 512 meg.