If it's a very pale yellow without an obvious sintered-bead structure (like foam coffee cups) it's most likely to be iso (polyisocyanurate), which will run ~R18-R20 in a 3" thickness. The asphalt-loaded paper facers is an indicator that they were probably marketed as roofing insulation at one time. If it's bead-board it's EPS (expanded polystyrene), and would run ~ R12-R13 in a 3" thickness. If iso, one layer should suffice, but if EPS you may want to double it up, lapping the seams by at least a foot for a better air-seal.
If it's iso it can't be used in contact with the ground, but is just fine for insulating a block foundation. It's still worth using a paint-on/spray-on type of masonry sealer on the interior face of the blocks before putting up the rigid-board insulation if it's iso, which can retain some moisture. (The asphalt facers are very slightly vapor permeable.) Waterproofing the exterior face of the blocks below grade is also a good idea.
Before putting up the iso, lap the ground vapor retarder at least 8" up the block wall an seal it to the wall with duct mastic (available at blue & orange box-stores) painted/troweled on about 1/8" thick. If it pulls away or has minor gaps (it's hard to make it perfect first-shot) come back and caulk or mastic the imperfections first. That way you'll have sheet plastic between the dirt and the foam, and it won't take on ground moisture. If your ground vapor retarder is in multiple sheets (probably is), lapping the seams by a foot and sealing the with duct mastic in the overlap works.
Then cut the sections of iso to fit the wall with ~1/2" clearances to make it easy to install. Use walnut-sized blobs of foam board construction adhesive, not standard construction adhesive, which has solvents that mess up the foam, and the bond between the facer and the foam (also available at box stores) every 18" or so and just smoosh it in place. Cut & glue sections for the band joists etc as well, leaving ~ 1/4" gaps around them. If the seams between foam sections can be butted super-tight they can be sealed with foam-board adhesive, but it's often easier to leave 1/4" gaps to be filled with single-part spray foam (eg. Dow GreatStuff or similar) later. Use foam to seal all seams & edges around the band joists as well.
If the crawlspace is all above grade (or less than 2' below grade) it's worth insulatinging the perimeter of the floor against the wall to at least 2 feet in, which will couple the building to the thermal mass of the soil, and the temperature of the uninsulated section will not vary much from month to month from weather variations, but might swing as much as 6-8 degrees between mid winter and mid-summer, but it'll always be above 50F (and may be above 60F most of the time) if you keep the first floor at 65F or above all winter. If it's more than 2' below grade (which it sounds like it isn't) the perimeter insulation isn't necessary, and just insulating the walls will be "good enough". But monitor the temp & relative humidity down there- particularly when outdoor dew points are in the 60s & 70sF (like this past week or so). (Cheap but decent temp & humidity meters from AccuRite are available even at Wal-Mart fur under $10.) If it stays above 70% RH for weeks on end in the summer a room dehumidifer should take care of it, but it's an indication of air-infiltration (maybe from outdoor air leaks in the rooms above, but the crawl will be at a lower temp==higher RH than the living space rooms in summer.) If it's below 55F in there during mid-winter cold snaps you may want to consider insulating the whole floor with InsulationDepot reclaimed stuff or Craigslist specials when the price is right. An inch or two of bead board EPS or XPS (eXtruded PolyStyrene) would be enough for the center-floor. With a plastic vapor retarder below you could also use iso, but crawling/walking around on iso will damage it (iso roof insulation has typically less than half the compressive strength of EPS or XPS.)
If you're putting heating/AC ducts in the crawl, be sure to mastic-seal all seams & joints before insulating them, and insulate them to at least R6.
Earth-coupled or fully conditioned sealed-insulated crawlspaces definitely work, and is absolutely the right way to go if you have plumbing or heating runs in the crawl. Like the guys at Building Science Corp point out, a conditioned crawlspace is really just a very short basement, and if you've insulated the walls you've protected the joists by keeping them warmer & drier, with little or no condensation potential for getting rot or mold started.