I would say go with the mini-van. Lots of room, plenty of space to store stuff, but the gas mileage is pretty good - I took a Voyager out to DC and got around 24-25 MPG.
Amazing Trip. Streets and Trips puts it at just over 3000 miles from Annapolis to Sacramento. With gas at $3 a gallon, should run you between $350 and $400 on gas alone.
Here are the tourism sites for all the major spots along US 50. Hope you have a great trip....
Washington DC: http://www.washington.org
Winchester VA: http://www.visitwinchesterva.com/
Clarksburg WV: http://www.escapetothesoutheast.com/WV-DMO-ClarksburgCVB.asp
Parkersburg WV: http://www.parkersburgcvb.org/
Athens OH: http://www.athensohio.com/
Hopewell Culture NHP: http://www.nps.gov/hocu/
Cincinatti OH: http://www.cincyusa.com/
North Bend OH: http://www.northbendohio.org/
Versailles IN: http://www.ripleycountytourism.com/
Vincennes IN: http://www.vincennescvb.org/
Salem IL: http://www.gosalem.com/toursalem/
St Louis MO: http://www.explorestlouis.com/
Jefferson City MO: http://www.visitjeffersoncity.com/
Sedalia MO: http://www.visitsedaliamo.com/
Kansas City MO: http://www.visitkc.com/
Kansas Tourism: http://www.travelks.com/
Olathe KS: http://www.olathecvb.org/olathe_cvb/default.asp
Emporia KS: http://www.emporiakschamber.org/
Dodge City KS: http://www.skyways.org/orgs/fordco/dctourism.html
Garden City KS: http://www.gardencity.net/chamber/ctb/
Bent's Old Fort NHS: http://www.nps.gov/beol/
Rocky Ford CO: http://www.rockyfordchamber.net/
(best cantaloupe in the world!!!!)
Pueblo CO: http://pueblo.citystar.com/travel_and_tourism01.html
Royal Gorge, Canon City: http://www.royalgorgebridge.com/
Gunnison National Forest, CO: http://www.fs.fed.us/r2/gmug/
Curecanti NRA, Colorado: http://www.nps.gov/cure/
Montrose CO: http://www.visitmontrose.net/
Grand Junction, CO: http://www.visitgrandjunction.com/
Green River UT: http://www.discovermoab.com/greenriver.htm
Fishlake National Forest, UT: http://www.fs.fed.us/r4/fishlake/
Nevada Tourism: http://www.travelnevada.com/
Ely NV: http://www.elynevada.net/
Eureka NV: http://www.co.eureka.nv.us/tourism/tourism.htm
Fallon NV: http://www.fallontourism.com/
Carson City NV: http://www.carson-city.org/
Lake Tahoe, NV/CA: http://www.bluelaketahoe.com/
Placerville CA: http://www.comspark.com/eldorado/
Sacramento CA: http://www.sacramentocvb.org/
California Tourism: http://www.visitcalifornia.com/state/tourism/tour_homepage.jsp