Ah, so I would not need a quad core CPU after all (with the condition that my PC is used just for running Minecraft and no other programs), but a faster one instead?
With my current machine, I am playing Unleashed in a public server and it runs quite well with two digit FPS, but it is a public server so my base follows the rules on the server, limiting 16 power-generators per chunk, etc. I also do not have quarries on the Overworld, but in a Mystcraft age instead. No SS spawner, just the regular one. Not sure how much is you refering by "developed" since I have not reach that stage yet.
Also I am not sure about the meaning of a kitchen sink pack, but I do have already messed around trying to create and modify a new or existing packs, just to try whether the pack will run well on me or not.
Well, if what I will need is a reasonable CPU then I might change my budget plan and consider to buy a new CPU, so the budget isn't too strict but I will not buy high-end stuffs, just the things that allow me to play heavy tech packs with acceptable framerate.
Thanks for your advices!
Kitchen sink pack = general pack with all sorts of different kinds of mods, as opposed to specialized packs which focus on a single theme (e.g. tech, magic, exploration, PvE combat, PvP combat) or specialized packs which revolve around a single mod. Packs such as Unleashed and Infinity are kitchen sink packs.
Just note that if you want a new CPU, you will need a new motherboard, CPU, and RAM - your current motherboard won't accept a current generation CPU, and a new motherboard won't accept your current RAM. You can keep the current GPU, hard drives, etc., or replace them if you have the money. You may or may not need a new power supply unit (PSU), depending on what CPU/GPU you get and the PSU you have.
If you get a quad core CPU, Minecraft won't use the extra cores but Java 8's garbage collector will use them, along with any background programs, thus you will still see some improvement over dual core (but you won't double your FPS over a comparable dual core).
And you will still need to get a 64 bit OS.