Hi mate. 512mb is a bit small memorywise for graphics. I think a new rig is on the cards anyway at some point so start saving. In the interim I would consider the SSD option as a stop gap. Just know it is delaying rather than avoiding an upgrade.
Having said this, to be devils advocate again, consider spending money on a good display and maybe a windows rig there i said it! But... Then you can have flexibility to swap out components at a much more cost efficient rate in the future. I could build a windows rig for £500 that would out perform my macbook pro right now.
The downside is of course you have to deal with windows...
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Ssd is a doddle... The hardest (and not really) is getting the screen off an iMac. The SSD is easy just do time machine, physical swap then restore from time machine.