The amp is definetly the most important part of your sound. IMO, a 15W Fender amp is total crap metal-wize. Here's the path Evil Ernie has took in his quest for a killer sound.
I had a Fender Deluxe 90. At first I knew fuck all about guitar sound so it was ok, but after a while I realised some friends had a god damn kick ass sound and I wanted to change my rig. The first idea I had was to change my guitar (Mexico Strat at that time), but I tried different guitars on my amp and the sound was somewhat always the same. I had no money for a great amp so I buyed a MT-2 metal zone to have a decent sound in the time I could get enough money to spent on my amp. This pedal made a drastic change in my sound. It works really good on a small solidstate amp. A year later I got my Peavy Triple XXX. I threw the MT-2 away, I don't use it anymore. Then I recently got an old Jackson Kelly with Jackson humbuckers (used for 300$, no need to waste thousands bucks on a guitar which will sound 98% the same). It is the guitar I currently use and I am pretty much satisfied with how my rig sounds now. I think that changing the stock pickups would be the next logical step for me, just to get slightly more definition and cutting edge in my sound. End of story.
I also warn you to make sure a Marshall amp is really what you want. As for the EMGs when talking about heavy metal pickups, Marshall amps are always the first hard rock amps we tend to look for. I suggest you to try the variety of excellent amps available before throwing your money for a brand name. I myself found the Marshall amps to lack gain. The Triple XXX is a beast.
Cheers.