Now I've got two pcs and one display. The display has multiple inputs, and a little button for switching among them... can I connect both pcs to the display (using different inputs obviously), and switch between them with both 'live'? Also, which would be better, DVI-I + VGA or DVI-I + HDMI ?

Yep, though depending on the monitor's connections you might need a combination of leads, eg one connects via HDMI and another through DVI as you mentioned. If you want all hdmi, you can get splitters for that sort of thing.

Yes definitely. I do this a lot.

Use DVI and HDMI. Get a USB switch too if you want to use the same keyboard and mouse.

Yeah I've got an old kvm switch bought years ago and never used that might do.

Yep, the hdmi/dvi switching works great. Trying to work out if the usb kvm (km actually), will be much use with both pcs being dual boot. If not I can do without.

Or use Synergy

That looks stupidly complicated.

(there probably is/are valid use case(s) for synergy; just not sure how it beats vnc or plain ole ssh. separate displays? really?)

The kv(m) switcher seems to be working ok, though I've only got the kb hooked up ATM (which uses the most real estate, obviously). I'm transferring a bunch of files from the old pc to the new, other than that particular scenario I will probably not be running both simultaneously very often. The switch just saves wear and tear on the usb plug & receptacle, and me having to remember (never a good bet) I suppose.

I have to keep a mini kb hooked up to the old pc via a ps2 converter plug since it never worked from usb until booted into an os (a defect of the k9mm-v design). After boot the shared kb works a treat.

It just occurred to me I can do this at work too with a headless server previously viewed over vnc on my mac. The server has video on the motherboard, so all I had to do was round up a vga cable. This is so much better than vnc. Mostly -- there's that whole business of two kbs-and-mice.