Sorry 'bout that, but I needed to mark your thread so I could get back to it.
First: you need to get the motif right. Having all of your marines on fire will be a little bit overboard, so probably, the best way to go about it would be to carry flame-based insignia through the army and leave the fire to important figures.
Where to get flame icons:
You can get the old Salamander Chapter gear, which includes several flame-based shoulderpads and gear. You can also grab some of the Mordheim Buidling sprues, which include lanterns and such. Bretonians from WHFB might be another good place to check. Don't forget, these guys probably have a fetish for Flame-throwers.
Easy Conversions (simpler than full-on fire):
consider chopping the vents off of your back packs and turning them upside down. Model fire coming up from them. This makes them look like pots of flaming oil or just really hot vents(duh).
You can also put some GS onto their armor to make raised 2D flames. This is fairly simple, just stick a thin sheet roughly the shape of the fire, then carave out little nicks from the border to make the individual tongues of flame.
Harder Conversions:
although only their bodies light on fire, you can always bend the rules and make a flaming sword here or there. These look really neat (I have several flaming weapons in my Dragon-oriented army), and are comparatiely simple to make since you already have a defined edge.
Other easy placed to put flame would be across back-packs and over shoulder-pad borders. The hard places are areas like hands, and the knees and elbows. Remember, it is the marine on fire, and not the armor, so you want the flames to look like their coming out through the joints, although a truly terrifying marine might generate flames hot enough that they DO eventually burn his armor.
There is a spot on the GW website that has a great technique for sculpting flames, but I don't know where to find it at the moment.
Possible Paint-schemes:
I know that you wanted suggestions on converting, but since I suggested not 'lighting' every model, you probably want to make fire visible on other models. I personally envision black armor (practicality: if it's balck, it won't matter if it's charred) with white and blue flames moving from the edges towards the top (from wrist toward elbow, from ankle towards knee, from waist towards chest etc.) Hot-rod flames would make good insipiration.
Or make the armor white, with white and blue flames. This would look really cool if the flames were one with ink-washes and then the entire armor given a gloss varnish.
well, I hope that helps you out a bit, good luck, and post some pics when you're done man.