the worst option of all the above is to get "additional 512Mb ram", u can very trivially run into issues and face BSOD's, so dont mix them up, also mention what ram have you got right now.
i would suggest u replace it ad get 512 Mb ram, your computer will not benifit from more ram than 512 Mb, 1Gb might increase the performance very slightly, but not worth actually...