[LAU] help for building new desktop pc

Hi list, I'm looking into building a new desktop, and possibly buy a new sound card, and I'd like some advice. Right now I'm using an Acer Aspire laptop, with an intel core 2 Duo T5500 - 1.66Ghz and 2gb of ram, and a Rig Kontrol 2 USB sound card. What I'd like to do is have one or two audio inputs, apply [possibly lots of] real time effects (like rakarrack and ladspa), have some samplers/synths playing, maybe a loop program, and record to disk. I probably wouldn't be doing frequently all these things together, but I'd like if possible a system that could, if needed, handle it. With my current setup even parts of this is very difficult (especially the real time effects part) and gets me plenty of xruns. Am I correct in assuming that the source of xruns is double: from the soundcard chip when the jack daemon is invoked with low latency settings (possibly too much for said chip) and from the CPU when there is too much DSP going on? I'm asking this because, being on a budget, I'm wondering if I'd be better off with keeping my Rig Kontrol 2 and getting an i5 2500k CPU or getting a lesser CPU but a better sound card... I know it's a difficult question with no straight answers, just wanted to hear some opinions. And while we are at it, do you have any other particular suggestions for building a desktop with my audio needs? For example is overclocking ability something to search for in a CPU? Online there is of course an overwhelming quantity of info, but most is targeted to people building computers to play games. best wishes, renato