If you just use the calculator as it is without any downloaded software, you probably won't notice any difference or that the plain G is actually a bit more spunky (turns on quicker, no hesitations, which is due to 32K RAM vs garbage dump and longer memory checks with the 128K RAM).
32K is plenty of RAM for personal programs and solver use, if you don't do much in the way of customization.
The G+ has the same 128K as your GX, but no expansion. The G+ sells for $75-100 generally (or did, I seldom check ebay anymore). You might also consider the new Hp48GII, but buy first before you abandon you 48GX (if you hate the keyboard or lack of the nice large enter key).
Do a quick memory check on your 48GX. If you have used over 20K or 25K (and have had it for years), you probably won't need the extra RAM. However, if you have used over 30K of RAM, you would be out of RAM by now on a plain 48G (of course you could have just loaded you 48G up with variables that you wouldn't miss if you did a reset).
Have you ever made use of any downloaded software? This is a great resource and you might visit hpcalc.org to look over what is freely available. Once you downgrade and then want your old 48GX back, you may pay even more for a 48 in much worse shape than what you already have.
A 48G has just enough memory for personal programming and one to two programs depending upon complexity.
A 48G+ has 128K RAM which would allow you to add CAS and some sophisticated programs.
A 48GX has 128K RAM plus the ability to add another 128K RAM to port 1 and 3.5 Meg to ports 2-30 (at 128 K RAM per port). The most common memory config is a 128K and a 1 Meg card. Klotz (in Germany) has these for about $40 and $70 respectfully. This option will not be available if you do not have a GX.
While you may make $100+ in this quick exchange you may or may not end up with a good 48G replacement (ie you will be at the mercy of another ebayer's description). That is why I suggest looking at the new 48GII. However, be warned, its keyboard isn't of the same quality of the older 48G series, though it is a big improvement over the recently discontinued 49G (which was replaced by the 49G+)
48Gii is a 49G+ with only 80K RAM and a serial port. No large enter key, same keyboard layout as the 49G+.