I think you've been using your C2D for 5+years?
Should be quite slow + short battery life.
I think it is very normal you want to buy a new one, and 不算亂洗錢。
About the ram...how many ram you are using now?
Even though T590 only have 1 slot, 8gb on board + 16gb free slot = 24 gb should be enough for most application, except you clearly know you need more than that.
Just quickly buy the laptop, then focus on study/work.