It's not that hard.
Usually the first digit marks the generation... and the second digit marks the
TIER. (The lower the digit the lower the tier... AMD:1-5 = budget, 6-7 = mainstream, and 8+ = Enthusiast... nVidia:1-4 = budget, 5-6 = mainstream, and 7+ = Enthusiast)
So with that in mind:
AMD: 46xx is much faster than a 62xx.
So yes, your GT 43x would be slower than a 96xx GT... but given the number of generations that have passed... it's not way slower.
nVidia: GTX 460 is much faster than a GT 520.
(nVidia also drops the "X" in their slower cards)
Get your games from GOG DAMMIT!