The best way to find out is honestly to just go ahead and try it! There's no reason that your system shouldn't be able to handle it assuming that the pack you're using doesn't have a ton of optional features that eat up system performance. And yes, it really does depend at least in part on the pack as even a 16x pack can be made to eat up loads of system resources with animations and CTM and so forth.
So there's no harm in just giving it a whirl and seeing how it behaves. You can always tweak the performance with Optifine to try to find anything that's taking up too many resources. In a worst-case scenario where the game becomes unplayable you can always just close Minecraft and delete the pack from your /resourcepacks/ folder to eliminate it without having to go back into Minecraft. That's not likely to happen, of course.
I'd turn that question back on you. Can your PC run the 128x pack of your choosing? Try it and find out!
Also, I have 7GB of ram dedicated to JAVA MINECRAFT.
If you don't need it to run, this is actually a bad idea. Java as a platform does some weird things so giving it way more memory than it needs can actually make your game run slower. Yeah, it's weird but that's Java for you. So if you can play with less, I suggest doing so.