Marshall JCM800 or 900 or 2000 (combo), what's the difference? I have about seven hundred to spend around Christmas time, might buy a new amp. Currently I have a standard 10W microcube copy, it suits me fine, I use my own pedals for my distortion and other effects. Would you think buying a JCM be worth it?

I play hard rock / hair metal. I have a HSH strat.

Try one out, there's always a chance you can find an awesome deal. Found my DSL100 halfstack for 850 bucks used (head AND cab) and I'm loving it.
Epiphone Les Paul Standard Plus
Jackson King V
Jackson Kelly
Handmade Gibson Les Paul
Marshall JCM2000 DSL100/JCM900 1960 Lead cab
800: standard Marshall tone. Needs a pedal to boost it into metal tones but can do any regular rock tone fine. Almost every hard rock and classic metal band has used 800s at some point. Really basic controls, no channel switching. If you want to be able to switch between clean and distorted tones easily then do not bother with one.

900: there's a few different versions of this so be careful what you buy. The 4100 head is the main one though and is most comparable to the 800. It's basically an 800 but with a second channel added, one that has a tiny bit more gain and a slightly thinner tone than the regular channel or 800 has. So this can do clean/distroted switching, if you set it up right. It's also quite capable of getting metal tones by itself, although these can be a bit noisy. The cleans and the lightly distorted tones can sound a little 'cold' and flat. Think of the 900 as being a 800 but with a boost pedal built in. Some versions of the 900 don't have proper two-channel switching though, some have reverb and others don't. Double-check whatever you buy.

2000: the 2000 is like the 900 in that there's a few different versions. The TSL line has 3 channels and the DSL has 2 channels. I use the DSL myself currently. The TSL has a little more gain to it and is a little bit more flexible but the cleans really don't sound as nice. So it's kind of like the difference between the 800 and 900 again. Each channel that the DSL and TSL has has two voicings, a lower gain one and a slightly boosted one. So the DSL really has four core tones and the TSL has six core tones. Plus they have extra bass boost switches and a mid scoop switch. The TSL's bass boost and mid scoop don't effect one of its channels, the DSL's two switches do effect both of its channels. The TSL also has more EQ controls. So the TSL gives you more options but the basic tone isn't as nice. The DSL sounds nicer for what it does but is a tiny bit more limited in terms of controls. I don't remember whether the combo version of either is missing any features. I used to have the DSL combo, which overheated. This is a common problem with them. Now I use the DSL100 head. Tone-wise I couldn't be happier.