First off, you can't upgrade an iMac graphics card. They're special form factors known as MXM. Theoretically, if you want to be extortionist prices for a better card that came from a similar model to yours, it could be done, but I wouldn't advise it.
As for Lion being necessary... I'd say no, at least not right now. If you have 2GB of RAM, then you have a Late 2009 or earlier model, so you'll probably be looking to upgrade in a year or two, which will probably coincide nicely with 10.8's release and the retirement of 10.6 support.
Otherwise, if nothing in the new features list of Lion really jumps out at you as a must-have feature, feel free to keep waiting it out.
My iMac with an Intel Core 2 Duo processor runs with OS X 10.6, or Snow Leopard. Been holding off on upgrading, unsure if Lion is "necessary". Also, I have 2 GBs of RAM, and I do lots of graphic projects. Should I also upgrade those too?

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