The RAM on the video card isn't the most important aspect when it comes to its performance. If you want to run everything on super high detail, you'll probably need 512mb of memory on there, but otherwise the textures can probably be compressed by turning down the settings, and 256 could work just as well.
You can check around online for benchmarks to compare various cards.
www.tomshardware.com is one place (some people don't trust the site, some swear by it, so idunno), but you can ask around and probably find others.