I've just joined a covers band playing mostly rock and a little punk and I guess it's time to upgrade from my current Vox VT30 (which I love to bits) but the little 10'' speaker isn't gonna cut it. I was confused anyway so I've been trawling here for a few days but I'm more confused now than I was before I started!

I'll be playing in medium size clubs and bars of perhaps 150 to 200 people so I'm guessing 50w of tube or 100w SS will do.

If I go for a stack do I get a 212 or 412?

I'm after crunch rather than full on distortion or cleans. I've been looking at the Marshall MA series both stack and combination and also the fender Mustang (the MA's y are getting shit reviews here tho!)

Ultimately I will play before I pay but can you narrow it down for me, I have about £600 to spend?

As an alternative, I can get hold of a modification for about £150 that puts an "out" jack in my Vox allowing me to wire it up to a 412 cab but how loud will this go?

If you are going to gig, then you need a bigger amp than your Vox. How much can you afford to spend? What kind of music are you playing? How many people are in your band? What kind/size of amp is the bassist using? Are there any other guitarists in your band? If so, what do they play through? And how loud is your drummer?

200 people is not a big gig, so unless they are spread throughout a huge venue, 50 watts should serve you well. For your gigs, you would probably be best served by a 2x12 combo. Others will recommend a 1x12; I just prefer a 2x12 in a combo. Most good 2x12 combos will have the ability to run a separate cabinet, so you can always add on without buying a new amp. If you have your heart set on a halfstack, a 2x12 cabinet would be fine for a 200-person gig.
"Maybe this world is another planet's hell?" - Aldous Huxley
It shouldn't cost you 150 quid to put an external speaker jack on the Vox, you can do it yourself in about half an hour (or even less) with a drill, some solder/soldering iron and about 4 quids worth of parts. Don't even need the drill.

For your alternative, look for a used JCM900, does punk very well and will do rock alright aswell. Look for a used 2x12 aswell and pair it with the JCM.

If you want a combo, look into the Laney VC-30 (if you prefer Vox-y tones) or VHT Lead 40 (for more Marshally stuff).
Cheers for the replies, I'm the only guitarist and I play mostly rhythm with the odd lead break and we play stuff by the KOL, Stereophonics so I'm after crunch but we do also play stuff like Undertones and Blink182 so I'll need something that'll cope with a little higher gain. My main guitar is a Blacktop Strat (2 humbuckers).

I'd like a stack but if I only need 2x12 I will consider a combo if it saves some cash. I have about £600.