Yes, as long as the card is a PCI-E card. The higher up the card the better it will run the game. If you cheap out your gaming will suffer. For Nvidia, I would recommend a 8800, 9600 and above. On the ATI side you could get a 4770 and be set (runs $100US, not sure what it would be there). A 3850 512MB should run Sims 3 just fine, though, so anything better than that (3870, 4670, 4770, 4830, 4850, etc.) would be fine.
As for your RAM, I am not familiar with Oz e-tailers but I would avoid eBay for components. You can get a 2x1GB set of DDR400 for about $60 US, so that would probably be $75 for you (possibly more). Check Newegg to see what brands are reliable.
You have a Celeron processor. A Celeron is basically a gutted Pentium in that it has half of the on-board memory cache, thus making it much less efficient than a P4 (which is pretty hard to do). A Celly works, but it is equal to a slower P4 because of the lack of cache.