I currently have a crate gt1200 half-stack. it was the cheapest i could find and i sprung to buy it. But now as my band is getting more serious im looking to upgrade to something a little better. I want to get either a 6505 or 5150, and then im not sure what cab. Which should i upgrade first? and also any reccomendations on cabs?

Im in a melodic hardcore band, sort of like the ghost inside, or hundredth. Here is one of our songs http://soundcloud.com/the-calm-before/live-today
Get a head since you have a cab. Crate amps are not that great except for the BV. What is your Budget and Location?
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Definitely head first. If you're in the US you should look for a used Peavey as you already mentioned. If you're in Europe you can get an ENGL Fireball for the same price and while the Peavey has a nice lead tone it also has a boatload of very stupid faults (extremely noisy, tends to get feedback, unable to deliver tones without a boost, stupid outdated footswitch format, HORRIBLE cleans) that make the Fireball a much better choice imo.

Cabwise I'd go with a Zilla if you're in Europe and a Mesa if you're in the US.
Ibanez RGT6EXFX -> Ibanez TS9 -> Korg Pitchblack -> Peavey 5150 II head -> Mesa Rectifier 2x12 cab
also Avatar and White Box cabs if you're on a tight budget. If you're not then Port City or Mills. Get the head first as it will make the biggest difference, but when you get a new cab it will shine. 6505 would work great for TGI thats what they were using when I saw them a couple years ago. If you want better cleans but a similar amp then I really liked the 3120 (I prefer it to the 6505 but thats just my personal preference). Maybe check out a Carvin V3 as well. Also heard good things about the Carvin MTS and they go dirt cheap. Lastly, for a cheap peavey amp that sounds great I suggest a Peavey Ultra. Ive seen a head go for $150 and a bunch for $200 or so and they are nice with lots of gain and much better cleans than a 6505.

Edit: This is all assuming you are in the US so if not I can't speak on used prices elsewhere.
Ibanez SIR27
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Mesa Boogie Roadster 2x12 combo
Cmatmods analog chorus, phaser, tremoglo, signa drive, butah, and deeelay
walrus Audio Descent