Is there some way to find out how much "power" each card has? Like I have know idea that difference between varying Geforce cards. I suppose the older ones are weaker compared to the new cards.
Yes...older is usually slower than newer when you stay in the same range (i.e. low-end, mid-range, high-end). Higher numbers are faster than lower (most of the time...). As a rule of thumb, performance doubles from generation to generation (ignoring refresh designs like 4870->4890 and similar).
It's hard to tell from the numbers what to expect and it depends on your personal view of things. To me, as a hardware freak, everything below upper mid-range isn't even worth looking at in terms of performance, but that doesn't mean that you can't play Robombs for example on an Intel onboard chipset.
IIRC, you posted in the Javagaming.org-thread that you have a GF4MX...it doesn't get much lower than this. The GF4MX is nothing more than a slightly improved GF2. It doesn't even offer shaders and has 2 texture stages only. If you design your game to run fine on that card, it will run on anything.