I have tried Aperture on the same (low-end) machine with both 2GB of RAM and 8GB of RAM. The difference is huge; with 2GB it is unusable, with 8GB it is OK.
So I would suggest that you first go through the memory upgrade and after that see if you still need more power.
Edit: This is an old question, and hardware has advanced a lot in the recent years. Others have already suggested that using an SSD could help a lot. Based on my recent experience, this is indeed the case. Moreover, SSD is already so cheap that you can often afford to put your entire Aperture library (with masters) on an SSD drive.
I just wanted to add that you do not need buy expensive Thunderbolt SSD drives; there is a much cheaper option: USB 3. You can simply buy an inexpensive SSD drive and a good USB 3 enclosure (e.g., this one). If you have a modern Mac with a USB 3 connectors, this combination should give you a very good performance in comparison with old-fashioned hard disk drives.