Both of the tasks you mentioned require lots of memory, alot of the new games out are recommending 768MB of ram for optimal play, with that in mind i would recommend 1GB of pc3200 or pc3500 DDR, you will need 2x512 modules.
If you have any further questions, i will be around.
First of all many AMD dual channel boards have only 3 dimm slots (not sure about intel boards...don't follow the newer ones much) so thats a problem right there; furthermore it is always good to have some extra dimm slots so that in another year (or whenever you next upgrade) you can get a probably buy then cheepo ddr 3200 or 3500 for dirt cheep and then you would have 1.5gb where as if you had the 4x 256's you would have to remove one of the modules that you paid good money for and not use it.
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