Ram computer memorie question?

okay, so i play guildwars on my computer, it only requires 512mb of ram i believe and that is what i have installed, believe me it runs very slow, often freezes up, so on our laptop we have a 1.60ghz amd processor with 1gb of ram and a fast dsl connection, on my desktop i have a bit better processor, 1.82ghz processor and a 512mb stick of ram, i can upgrade to 3gb and on our laptop the game runs pretty much perfect, very little lag if any at all, but on the desktop like i said its terrible, so i came to the conclusion its the ram because i have a 256mb ati radeon x1300 pro agp video card, brand new, figured that would fix it but it just gave me amazing graphics, didnt speed up nothing but fps, so should i upgrade to 1gb of ram memorie to fix the lag problem?