If you can tell us a bit more about your system it would help us to advise you more clearly but in essence your main aims should be to maximise any RAM useage (4Gb is the maximum for a 32 bit OS and I would have thought up to 16Gb if you are running 64bit Win 7 and your motherboard will take it). You have identified the i3 as being a limitation so you could go to an i5. If you are streaming video then I would check to see what graphics capability you have. If it is a chip on the motherboard then I would add a dedicated PCIe graphics card.
HTC U12 Plus review: Hands-on