if I were you, I would wait until online discount when christmas arrives.
currently if you allocate 70-80$ more on your budget you could get an RTX instead (if you want to stick with the brand.)
memory bus doesn't really matter since it's a weaker card, I don't think you will run it at 4k, high fps and stuff. if you were, it would matter but if not, dosent really do much, especially on weaker cards.
if you don't want to spend more than what you want right now, go with RX since it supports Ray-Tracing, if it's your thing. but not the RX 6400, it's perf is dog shit
same price you get a 6500 XT still 64bit bus.
100$ (if you look well on amazon, newegg) more you get the RX 6600XT with 128bit bus if it really bothers you, 8GB DDR6, and more, basicaly, it's like double perf than RX 6400
long story short, if you wanna stick to your plan, both card perform the same, you can get more out of the RX on certain scenario some others will perform better on the GTX
GTX you don't have raytracing.
RX you do have ray tracing, better to futur-proof your PC in the long run.
PS: I've been testing alot of CPU/GPU combo lately at work for perf gathering, I know my stuff