Though I grew more partial to the Ensoniq ASR-X Pro, the MPC is handy. Alot of people are gonna tell you to use a computer to sample and the truth is they're right in alot of ways.
If you're looking to just snatch loops off old songs then software like Acid is probably better for you because its so much easier to sample when you can see the wave form of the kick or snare to cut it at the right spot and even tempo matching is way easier. They didnt use a sequencer to time everything on point for most samples you're gonna take so the tempo of the sample you take from early in the song will often be a decimal point or even a whole bpm or so off in the later loop of the song. Oh, and if you are looping and you're new to it you wanna make sure you dont cut the space after the last snare before the 1st kick of the next bar, cut it at the last moment before the 1st kick of the next measure comes in and its best to get a 4 bar loop or AT LEAST a 2 bar because a one bar loop can become too repeatative too quick.

With the capabilities of the MPC its better to sample a sound rather than a loop. This is what makes it better than software w/o a midi keyboard. Sample just a horn and you can play that sound as if you are the original composer... this is what the MPC still holds its ground with. You can sequence it out or (my favorite way) play it live to a click track (metronome)and then quantize it later. I also like the live drum sequencing feature too. When taking drums do the same as I mentioned with sampling a single horn note from an old soul or jazz record. sample all your drums indiviually from records and sequence them yourself (thats how Dr Dre did the drums for the Chronic. Old Skool songs are good for this especially with getting a variety of good 808s. Alot of the drums on The Chronic were from The Bridge is Over by KRS1 (as you get into producing with breakbeats for awhile you develop an ear for recognizing classic kicks and snares). Save all these sample sounds and drum kits into back up files for later use when that right snare is needed.
I got a ton of kits sitting on Zip disks all over my production desk for my ASR-X Pro. Pre-sampling will give you alot more options.

One last thing... try to sample as many long notes as possible as sample the air that drags on after the drum or it will sound too tight.

hit me up on myspace if you have any more questions incase I dont reply soon enough for you on here.
myspace name is "ROMEssiah" just like on here

ROMEssiah - "Inappropriately Touching" album coming early 2008! Deadly Mix Productions -- R.I.P. King Joe