I'm running Win 7 with 2GB of RAM, if I'm not running Fancy graphics with the water shader mod I get about 25-40FPS. My sister has a Mint netbook with 1GB of RAM, it's playable but only on low fog settings - this obviously means 1GB is a no-go on Windows. I'd recommend getting 4GB if you can, though. And try and go for the least amount of individual RAM cards so you have more slots for further upgrading, as long as it's the same brand/model it should be fine. I'd just get a 1GB card, and if you have room maybe another, and if you have money replace your two 256's with 1GB's. 4GB is ideal for Minecraft, but 2GB works.
It's a different story if you want to run a server on your own box. 3GB is the minimum for a half-decent server.
I'd suggest getting a new computer, really. DDR2 memory is starting to go up in price quite a bit (similar how DDR went up after DDR2 was the standard for a while) and pouring money into a hunk of junk is never a good idea.
But if you really have no other choice get 2GB. You don't need 4GB, or rather, you couldn't handle 4GB worth of programs with the processor you're using.
I built my own computer in 2012, Specs are: Intel i3-2100, Amd radeon 5570, 8GB of ram, 500GB Hard drive, Antec One case, Z77A G43 Motherboard, OCZ 600W PSU. Windows 7 64 Bit.
Also I have a youtube channel https://www.youtube.com/channel/UCc9Bi9r4UPZbAWyuRKBJeoA
I like to play games on steam like Garry's Mod and robocraft.