Sorry but I cannot answer the card vs. memory question. Many apps (e.g. browsers) do leak memory but I have not identified the Aperture app as leaking memory so much as liking memory. 🙂
Aperture is a heavy pro graphics app that is very demanding of hardware. Your Mac's CPU, GPU, i/o and/or RAM all are probably limiting to Aperture so we need to do what we can to optimize.
Some generic advice I posted in another thread is listed below. In your case I would pay special attention to RAM optimizing: restarts and running Aperture by itself in the short term, adding RAM in the longer term.
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• Set Previews to only be built manually, and only tell Aperture to build Previews when you are not otherwise editing in Aperture.
• Keep the internal drive underfilled for speed, which sooner or later usually requires using a Referenced-Masters-Library with Masters on external drives. Back up originals before importing into Aperture or any other images app.
• The Aperture Library should be on an internal drive.
• Turn off Share Previews with iLife and iWork.
• Remember that HDs slow as they fill and keep the internal drive underfilled. Approx. 70% full maximum is a good guideline.
• If page outs increase significantly during operation try to run Aperture by itself. On my C2D MBP with its max of 3 GB RAM I always did a restart prior to a heavy Aperture session to clear any memory leaks and make sure no other apps were open. Browsers in particular will often suck RAM Aperture would otherwise be using.
IMO all Aperture users who can should routinely bump RAM to at least 8 GB.
Activity Monitor's RAM reporting can be misleading. On Activity monitor check Page Outs before and after a work session. If the number of Page Outs increases significantly, you would benefit from more RAM.
HTH
-Allen