So you have two of those sticks? How many available slots? Ram should be updated evenly, so depending on what room you have, you could either get 2 more 1gb sticks, or two 2gb sticks.
I wouldn't really want to stress your power supply more than that unless you wanted to get a new one. This really only makes sense if you want to use those parts in your next PC though.
Anandtech has a good June comparison of lower power cards : http://www.anandtech.com/show/5969/zotac-geforce-gt-640-review-