With regards to HDMI/DVI: It's not possible to run HDMI at 2560x1600. Ever. So, even the rare 30" monitors that have HDMI-in will scale the resolution down to 1920x1200. With the exception of the troublesome Apple mini-DisplayPort to DVI adapter, all DisplayPort to DVI adapters will also fail. So, the only alternative to dual-link DVI output > dual-link DVI input for 2560x1600 is DisplayPort output > native DisplayPort input (and there are only one or two 30" monitors with that for now).
With regards to buying a new PC to lower power consumption: as lm pointed out, that will virtually never pay for itself. Nor is it good for the environment, considering how many resources are wasted and how much power is spent to build that one new computer. (The same applies to cars, for all of you Prius-buying people!
With regards to what to do with the nettop: sell it or return it to the store, unless you've found a good use for it. That "crazy smoothness" in your music doesn't make sense. You might just be used to noisy laptop sound outputs. If you want to get rid of that, buy yourself a cheap external DAC/amp or sound-card (USB or s/pdif / optical).
With regards to Ubuntu being lower wattage and sounding better: that makes no sense at all. Even if there IS really a perceptible difference between sound quality in those two OS's, there is a different explanation for it - e.g., differences in WinXP's and Linux's management of sound.