A laptop would be in the cockpit where a larger screen
makes it more useful.

Unless you have an enclosed cockpit that's not going to be a very good idea.
Laptops have neither the durability nor visibility that lends them to being
used in an outdoor setting like an open cockpit. And even when you're in an
enclosed space the screens usually aren't very visible in sunlight. Not
that you can't use them, but unless it's something like a panasonic
ToughBook (or equivalents) it's a waste of time.

I know of no radar gear that works with a PC as it's display.

Am I dreaming...or is this sort of technology available?

For sensors other than radar, sure. Maretron sells a USB black box that'll
interface your PC to an NMEA2000 network. I've got NMEA2000, Seatalk and
NMEA0183 all running in our boat. But I rarely use the PC as an active
member of the network. The chartplotter (a Raymarine E-80) generally does a
better job at the helm. The interface, screen and buttons are just better
suited for it. I've got one GPS on seatalk, along with the autopilot, depth
and speed and the fuel flow and engine data on NMEA2000. The laptop pulls
GPS via NMEA0183 from the chartplotter. Weather comes from Sirius
satellite and is fed to the chartplotter via Ethernet.

I do use Rosepoint's Chart Navigator on the PC for route planning or more
detailed location searches. I don't currently pull NMEA2000 data into the
PC. If/when I do it'll probably be using Maretron's interface.

Personally I prefer the built-in screens instead of laptops, PDA or cell
phones. When the going gets rough the built-in equipment is a lot less
likely to get ruined (or splashed overboard).

-Bill Kearney