Besides a new video card ram would always help. Though I do not know your machine, mine which is the HPa6300f shares ram with the gpu on the motherboard. That is unless you use a PCI-E video card where system ram will also therefore be shared with with the video card. Hence more ram is always needed to make things run smooth.

After a further look, I would suspect the video card may not do Aero at all. Unless it supports (has internal shader hardware) among other things it will not support Aero. The processor certainly would run Vista but, thats not the problem with Aero. Your Ram numbers in performance may show a little climb up the chart but, not much else. If you are wanting to just run Vista without Aero and all, your machine should do just fine.