(MONEY Magazine) – THEY'RE ABOUT THE SIZE OF TODAY'S CDS BUT HOLD less than a tenth as many songs (one per side) and have a fraction of the sound clarity. Still, 45-rpm records are proving to be sweet music to nostalgic baby-boom collectors. For example, "It's Too Soon to Tell," a rare single by rhythm and blues band The Orioles, cost less than $1 when released in 1951. Now it commands as much as $3,000. The records most likely to approach that price level over the next 10 to 20 years are extremely rare recordings by artists popular with both boomers and their kids, such as Jimi Hendrix and the Beatles. If the 45 comes with a sleeve in pristine condition, it may fetch twice as much as the record alone.

Hot: Obscure records cut by 1950s and 1960s artists before they became popular; also promotional and test copies distributed only to disk jockeys, stores and recording studios (most are marked "for promotional use only" or "not for sale" on the label).

Not: Post-'60s mass-produced 45s; classical music.

Entry level ($10 to $50): Moderately rare '50s and '60s singles in good condition by popular artists such as Bing Crosby and Marvin Gaye, valued at $10 to $35; early 45s pressed on brightly colored vinyl (mainly yellow, red or green) to catch the eyes of disk jockeys, such as Marvin Gaye's 1976 "I Want You" (at right), valued between $25 and $50.

Mid-range ($50 to $250): Singles featuring rare versions of tunes that later became hits, such as a 1970 Derek and the Dominos cut with "Tell the Truth" recorded twice as fast as it should be; a copy may fetch $50 today. Also, limited-edition singles by post-'70s bands, such as REM's 1981 Hib Tone recording of "Radio Free Europe," valued at up to $150.

High end ($250 to $2,000): The rarest singles from the '50s and '60s by artists who later became big stars, such as Otis Redding's "Shout Bamalama" from 1960, eight years before his first national hit; popular musicians' test singles, which were labeled as such and typically distributed to no more than 50 people in the recording industry.

Where to buy and sell: Used-record stores; flea markets; record conventions in major cities advertised in the biweekly magazine Goldmine (Krause Publications, $39.95 for one year; 800-258-0929).

Resources: Goldmine's Price Guide to Collectible Record Albums (Krause, $19.95); The Official Price Guide to Records (Ballantine, $20; 800-800-1462); the monthly magazine Discoveries: For Record and CD Collectors (Antique Trader, $21 for one year; 800-364-5593). --K.H.