Im going to play in a cover band and would need alot of pedals so ive been looking into getting a multi effects unit. One thing im not sure of is this....these units have lots of different amps to choose from so if i chose a twin reverb setting on the unit would it sound like a twin reverb even though its going through my marshall amp??
Not really but .... If you are running the Marshall dead clean you will get about as close as the unit can produce.
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
Every man and his dog seems to have a conflicting opinion about this, so here's mine...

I've been using a POD X3 Live for a while now; I absolutely love that thing. It's never let me down, and it does everything I need.

As far as amps go, I've found it varies a little from model to model; some are really close, and even respond with the same little idiosyncrasies you might expect from the real thing, whereas some just sound like what they are, like someone tried to copy that amp.

The general consensus is that you sacrifice a little tone for a lot of versatility.

Also, as a last little tip, I had a *lot* of problems when I transitioned from playing through my PA to a stack; the thing just sounded rubbish through everything, even some very nice ENGL heads I tried out to combat the rubbish-sound problem. The insanely simple solution is the bypass the preamp by running straight into the effect return, so you're just using the power amp. Now, I play through a Fender Metalhead, which sounds like a pile of rusty arse on it's own, and sounds absolutely beautiful, and tubey, with the POD.'

I can't speak highly enough of them, but to each their own.
I was looking at the Digitech RP1000. If your getting one of these effects units does the quality of the amp your going through make a big difference? I mean would you be just as well getting a cheaper amp with a decent clean tone?
I would say yes; The Metalhead isn't renowned for its tone, but it's a well put together amp with a huge power section. Whereas when I played it through a Peavey valveking (100 watts, all tube, but cheap), it still sounded pretty awful, power-in or not. So yes, if you're amp is terrible, it's unlikely a multi-fx will save it. Maybe bring it to a store and see?

I'm a Line 6 *****, and I know this, but if you can afford the POD X3 Live, it's a million times better. I experimented with Digitech back when I was testing them out, and the POD blew them away.