Upload & Sell: Off
As a bare minimum you should upgrade the external HDD case to one that has a FW800 interface. That will be significantly faster for bulk transfers though not for file access times.
How big are the internal and external HDDs ? Higher capacity drives might boost the speed just by being bigger - especially if you create two volumes on each one so that the first (fastest) half or so of the big drive is being used most. Even the slow half of an internal drive will be significantly faster than any HDD on a USB 2 interface.
For Lr to generate previews it needs to open the original raw files and do raw conversions to extract an image, and then scale it down as required. The preview will then go into the preview cache, and part of the converted raw data will go into the ACR cache. Performance will be better with subsequent edits if both of those caches are big enough to keep the data and also if they are located on a speedy drive - which means internal, and preferably an SSD - but for the first time a raw file is imported into Lr it will not be in either cache. The only way to speed up the process then is to have the raw files on the internal drive.
One option is to keep the newly imported files on the speedy internal drive until the bulk of the edits have been done on them in Lr and then use Lr to transfer them to a new home on an external drive later on. You can set that transfer in motion and then go and have dinner so that the time it takes will be irrelevant.
The only significant downside I can think of to importing to one drive and then relocating to another later on is than Time Machine will make additional copies of each file - one for each drive that the files ever live on.
If you're still running a small amount of RAM then upgrading to at least 4GB will help. More than that is nice to have but Lr will not improve significantly.
Finally, if you are running Lr as a 32 bit program then change it to 64 bit. I'm not sure how old your Mac OS is but Snow Leopard or later will handle 64 bit quite ok.
A new computer would be heaps faster and the retina screen would look a lot better too for photos. However, I'm waiting in the hope that they put a retina screen on a computer that can take multiple standard high capacity drives internally as user upgrades and has more thunderbolt and USB 3.0 ports available as well as a FW 800 port. Then even the external drives will effectively be as fast as internal drives and everything will be so much faster.