My PC runs on a 'single core' AMD Athlon, 64 Processor 3400+, 2.20Ghz, 300Gb RAM (older RAM). It could easily run Pianoteq while a host of other media task are running: TV, DVDs, Internet; (streaming Video content; YouTube, BBC iPlayer (or similar)), Norton running background tasks. The PC is about 7 years old and is starting to feel old now.
I'm sure any modern PCs specification running on a dual core and above processor, latest memory etc.., will make light work of Pianoteq. If your PC does struggles to run it, check the 'Sample Rate' and the 'Polyphony' count in the 'Options' menu and adjust it accordingly. Test it by hammering all the keys at full velocity, whilst using full sustain (head phones recommended at this point, if you're sharing the habitation) and keep an eye on the little '0%' found in the top left corner, near the 'Midi' controls. If your PC doesn't bottle-neck at this point you should be ok.
I don't know how much of the CPU Ableton live will use. I'd go for the best CPU specification (and Hard-drive space) that I could afford, because I find all the other elements easier to upgrade later.