Over the last several months I have been getting into blues, r&b, soul, and funk. I was recently able to get my hands on “Otis Redding’s Greatest Hits,” Sam Cooke - “Portrait of a Legend 1951-1964,” and Stevie Wonder - “Songs in the Key of Life.” On my computer I have various tracks by BB King, George Clinton, Koko Taylor, Wilson Pickett, and Earth, Wind and Fire. I’m sure this only touches the surface of what’s out there. What else do you think I might enjoy?
You need some early Bob Dylan - specifically Highway 61 Revisited
You also might want to delve into swing and zydeco - very cool music
I strongly, strongly recommend Louis Jordan and T-Bone Walker - both did Jump Blues - basically blues for a full swing band orchestration.
Get No Moe! the best of Louis Jordan - it sounds like the best party you’ve ever been to. Ever heard Joe Jackson’s Beat Crazy? He was doing his best Louis Jordan? How about the swing craze from a few years back? Same thing.
T-Bone Walker was the first electric bluesman - came from Texas - taught folks like Albert King and Gatemouth Brown, ended up in LA and just smoked. I have the Complete Imperial Recordings, but there are a number of great collections out there…
If you don’t have James Brown - 20 All-Time Greatest Hits - go buy it. Now. Just take your fingers off the keyboard and go get it. Now. Really.
Same with What’s Going On by Marvin Gaye. Buy it. Immediately.
Don’t make me keep going - I will spend your full budget.
It’s not entirely clear whether the you want general recommendations, or recommendations for the specific genres you mentioned.
There seems to be a lot of crossover in the genres I mentioned, so general, to start off with.
A lot of people left to go in the vein of the original desiderata. Lloyd Glenn. Hank Mobley (“Dippin’,” e.g.). Charles Brown. Don Wilkerson (with Sonny Clark). Jack McDuff.
Maybe it’s better to go deeper into your own record collection, really understand what about that music makes you dig it. No? You can learn a lot from listening to one record, provided you’re really listening (to the harmony, to the rhythm, to the melodic patterns or motifs which unfold over the course of a good solo).
There is something to the notion that listening to more records will increase your enjoyment, but if you understand the music to which you listen, it’s a lock that you’ll better discover whole swaths of music which may not be evident from the perspective of a merely casual – therefore inattentive – listener.
Well, duh! There’s ultrafilter’s extremely cool SDMB music project - check out his sig and let your fingers do the walking!!
Funkadelic: Maggotbrain. If you don’t already have this one get it immediately.
Also, I really like Al Green, and not just “Let’s Stay Together”. His greatest hits album is pretty damn good. I’d recommend it.
Superfly, Curtis Mayfield.
Eva Cassidy. Start with “Woodstock” (yes, the old Joni Mitchell song), “The Letter”, and “Ain’t No Sunshine” off Time After Time; “People Get Ready”, “Blue Skies”, “Fields of Gold”, and “Oh, Had I a Golden Thread” off Live at Blues Alley, “Imagine” off Imagine, and “Wade in the Water” off Songbird.
Gil Scott-Heron, who combined Funk grooves with Jazz harmonies and the sort of political commentaries that later grew into Hip-Hop. Very 
Also, Tower of Power (the album “Tower of Power” is probably the best place to start) and the Neville Brothers (check out “Yellow Moon”) are two Soul/Funk bands who tend to be underrepresented in most collections
Vouch. Pandora will turn you on to artists that you otherwise might never hear of. Once you find somthing that sounds interesting, a service like www.rhapsody.com (or Yahoo Unlimited or Napster or Virgin or, heck, there is probably a lot of them available) can let you be more specific about what you want to listen to. Once you get set up with something like Rhapsody, it’s like being in a candy store. You can probably listen to every artist suggested so far.