I used to support HP corporate level laser printers, my input:
1) what difference does memory make
The biggest thing with printer memory is you have to have enough to store an entire page worth of rasterized data at once. 300 dpi to 600 dpi increases memory requirements fourfold, and to 1200 is fourfold again. Additionally, if you have a duplexer you have to be able to store TWO full rasterized images (front and back). Color obviously requires even more than monochrome.
The more memory the better, though for most consumer printers (and consumer use) you won't need exorbitant amounts of RAM.
2) does print quality degrade over time like in ink jet printers?
As long as there is sufficient toner to evenly cover the developing roller in the toner cartridge (magnetic roller that runs the entire width of the page) there will be no image degradation. Shaking the cartridge redistributes toner, though newer toner cartridge designs do not benefit much (if at all) from shaking. Once you run out of toner you develop blank strips (where there is no toner to develop the image on the OPC).
3) does refilling toner cause damage or wear on any other part of the printer?
The problem with refill cartridges is typically not the toner, but the methods used to reseal the cartridge. The biggest danger is leaking toner, but occasionally clips used to keep a cartridge together can come off. If that happens you risk damaging the internals of the printer.
Additionally, cartridge components are not designed to last forever. Yes, they can typically last more than one refill cycle but they won't last forever. 90% of bad toner cartridges I dealt with were refilled/3rd party cartridges. Actual HP cartridges were rarely faulty, and when they were they were covered by HP as long as they had toner in them.
4) any recommended places to buy toner refill from? if ebay, which seller? most likely for samsung ml-1740
I don't recommend it, so I honestly don't have any suggestions. Look at it this way, a full capacity cartridge will likely last you several years (even on consumer level laser printers 3000 pages per cartridge is normal, corporate level printers have cartridges that last 10K pages). 3000 pages is 6 full reams. 10K pages is 20 reams of paper. That's an awful lot of paper, $50 saved on a cartridge spread over that many pages really isn't that much.
I won't go so far as to say that refills are a bad idea, but I wouldn't do it myself.
Viper GTS