I have to echo the Norseman. It's just a big question. Like "what is a good car."

    I think the people on this forum could just throw out the names of high gain metal half stacks all day long, ranging from the relatively affordable to the stupidly boutique. Bang for the buck, I still haven't found anything better than my Carvin V3 (for the money), but as with all things it's about taste, style feel, personal preference.

    To answer some of your more specific questions:

    EQ pedals are useful, but whether you're talking about a Stompbox EQ or a rackmount parametric EQ, you have to be pretty careful. You can seriously mess up your tone. And even if you think you have a sound that makes you sound like a METÄL GÖD (complete with umlaut dots) you can get into a live setting and find that your epic scoopiness gets lost in the mix. Yeah, that's advanced stuff, but you are taking it to the next level, after all.

    On the subject of compression, I love some compression. It's just hard to understand the value of it right away without having it explained right. For years and years I always just assumed it was about normalizing volume levels. You know, making the quiet stuff louder and the loud stuff quieter, so you have a more consistent volumetric ouput. While that is true, it's not the REAL value, at least not for me.
    For some reason, compressor pedals are all the rage with the Nashville Telecaster crowd. My guess is that they use it to supercharge clean attack so that all those chickeny little runs POP right out of the amp. Which is cool I guess. Me, I shred leads at super high gain, and my right hand technique is fast and light. I have my compressor set up to supercharge the attack on my picking so that all my hundreds of fast light little notes, even the palm muted ones, are banging hard against the 12AX7 preamp tubes and really just sounding killer.
    If you're mostly a metal rhythm player, compression still has value, but maybe not as drastic as what I use it for. But if you're playing Sanitarium or whatever and switching between clean picking and high gain rhythm, a compressor would be useful for keeping things consistent and "Pro."

    Finally I'll mention OD pedals, since that might be something that someone recommends at some point. I personally like an amp that gives you "Your Tone" with the right amount of gain straight out of the box. But some amps can benefit from an OD pedal, whether it's an Ibanez tube screamer, a BBE Green Screamer, an MXR Zakk Wylde pedal, etc. Keep in mind that these are not distortion pedals that give you your tone, you put them in front of a distorted tube amp to really make it crazy heavy. With some tweaking, sometimes I can give my Carvin V3 a little extra screaming juice by using a Zakk pedal, but you just have to be careful of the extra noise.

    Technically it's possible to buy expensive pedals that are actually the entire distortion tone, and put that into a clean channel, but I've never personally heard any that made me want to trade in a high gain amp.

    If you get to where you're amp shopping, the people here will give you specific opinions, just be prepared for a variety of reactions to any given head.