With Aperture RAM and CPU is less important than the video card (not to say they don't count, just not as much as the video card). It doesn't matter how buff your system is, if you have the low-end video card Aperture will crawl. The reason is that Aperture is video hardware accelerated.
The base video card on the tower is the GeForce 7300 and from the reviews I've read of aperture, that card is under-powered. You'll want to upgrade to the next one up, the ATI x1900. Unfortunately, the iMacs best card is the 7300 so performance may be rough especially when you have lots of adjustments going.
Download the trial and see if it'll work for you. I know the recent update to Aperture sped things up slightly but I don't know it that version is the same as the trial. When I tried it, the trial was 1.1 when 1.5 was out.
I think it's a safe bet Lightroom will perform better on your iMac than Aperture since LR is CPU and RAM-bound and does not rely on the video card.
What does actually happen, if you try out both Aperture and Lightroom in their trial versions. Will you still have access to the photos you worked on if you don't choose the software
Yes. LR will store everything in a file folder structure you can easily read. Aperture keeps everything in a package which you can view by right-clinking on it and selecting "show contents." However, I don't know how the images are structured inside.
I think metadata is exchangeable when you export since it's XMP-based in both apps but otherwise what you do in one will stay there.
When testing, set aside copies of some images from your main collection and import those into the programs.