From what I can make out, the GT610 is hardly an upgrade from the GT520 - when the 610 was launched, the comment was that it was no more than a rebadged 520. Perhaps you know different? Anyway, to your question: on graphics cards - like PCs - more memory is always good. But if you had the budget, I'd be looking at something with DDR5 memory rather than DDR3, as it's significantly faster.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been