I guess mouse is the way to go... I never want to block my CF or SD and specially in your case where you have SD and CF storage cards.
I have ask the map size question in this forum and the reply I got was little over 1GB for entire US. So with your 5GB HD you can cover all that... but I don't know what performance hit you will encounter while running the maps from HD. On the other hand you can load recently used maps from SD...
As for the GPS receiver you have two choices
1) Mouse
2) MMF (Multi Mode) i.e. both CF and Mouse
(2) is expensive but gives you everything you need while (1) is cheap and can fulfills most of the requirements but not all.
Recommendations:
(1) GlobalSat or i.Terk Mouse GPS
http://www.semsons.com/itmousgpsrec.html
(2) Haicom H1-303MMF
http://www.semsons.com/haicmulgpsre.html
For Comparative Analysis
http://www.gpspassion.com/en/hardware/gpslist.htm
Check the rating GlobalSat/i.Terk Mouse and Haicom MMF both got 'A'
Software
1) Routis / iGuidance
2) Mapopolis
If you can buy a bundle then you can get good deals like
Mouse
http://www.semsons.com/itmousgpswig.html
MMF
http://www.semsons.com/haicmmfgpswi.html
Plus you can get a 10% discount coupon if you become a member of GPSPassion.com (Which will cost you $10) this $10 will be well spent for support as all the software providers have their forums in GPSPassion once you become a member
Buy everything from Semsons.com they are good. Excellent service I ordered mine yesterday and looking forward for its arrival.
Lets do the math then
Mouse Bundle: $199.99
Discount: $20
Shipping: 4.99
-------------------------
Total: $184.98
Support: $10
Your Total: $194.98 (if you buy iGuidance you will get one map update free first year)
I hope this helps!
Good Luck