and using it as a separate hard drive for editing photos. Do you think it will be slow/laggy while editing with aperture?
What Aperture likes is RAM. Put in however much fits in your Mac and Aperture will run as fast as it can on that computer.
To answer your exact question, we do NOT have to guess and say "I think...". All you do is run "Activity Monitor" and watch the disk IO rate. Is it running at the maximum rate the disk can handle? If not then getting a faster disk will not help. If it is maxed out then yes a faster disk will help BUT only during those periods when the disk is as max speed.
Next take a look at the CPU. Is it maxed out at 100% per core? Same here. With Aperture much of the time it is simply waiting for input and the operator is the speed bottle neck.
If you just want fast vieing. Turn on preview mode (the yellow frames) and set preview image size to screen size. Aperture is very fast in this mode even on low end hardware.
The key thing with computer performance estimates is never guess when you can measure.
With Aperture you really only need to keep a fraction of your library on-line. The program is designed so that you download the images the active library on the internal disk and work on them there. Later you can off load the images you are no longer working to external hard drives or another computer. Apple's license even allows you to install on two computers (1 desktop, i notebook) because this is the way they expect you to use it. You will need a few external drives for "vaults". be sure to have at least two.