depends on how much you want.
2*512 sticks are £60 at crucial whereas 1*1gb sticks are £123
I have a mac mini and a macbook, both were 512mb which retrospecivly now i've upgraded both was painful.
Last year when ram was expensive I got 2*512 for tthe mini and it made in a different machine, much more usuable.
This year after the price of ram dropped i got 2*1gb for the macbook, and the makes the machine even better.
It all depends on how much you want to spend! But either 2*512 or 2*1gb will make a wonderfull difference to your mac!
Unfortunatly the bin is probably the best place for the 256sticks coming out of it! Ok well a cupboard then, you know what i mean, they are practicly usuless to sell on and no one wants them!
I bought crucial ram, which is good but you can get cheaper, although it's halved since the expennsive prices of last year!
http://www.crucial.com/uk
For a cheaper alternative try scan
http://www.scan.co.uk
Which seems to offer £54 and £102 so slightly cheaper, you'll have to check out delivery though!