I've been practicing on a GAX30 and i've got fairly decent by now. But it's a very basic guitar with ****e pickups. So i'm after a new guitar. Not anything overly fancy or expensive, but something that i will notice a good improvement with.

After looking around a few sites the ones that really caught my eye are the Epiphone LP standards or customs. How much better are they compared with my ibanez gax30? Are they good for classic rock? Deep purple, metallica, iron maiden etc etc.
An epiphone Lp should be perfect for the bands you mentioned. You might want to swap out the pickups later depending on your taste, but I dont think it would be necessary. I dont have any experience with the Ibanez gax 30, but I imagine it would be at least a step up from it.
Is there much difference in sound between the standards and customs? If not i'll go with the standard as they are cheaper.

Oh and another thing

I hear good things about fenders... but i have this voice in the back of my head telling me not to even consider them. I think it stems back to when i was young and i was bought a starter guitar pack, and the guitar looked like a fender strat... i think it was some yamaha strat copy, not sure. But it was horrid and sounded awful, and I think that experience has put me off ever owning a guitar that looks like a strat. Am i the only one this has happened to?