Even now, imperial guard and priests arn't that bad in melee. Even better, an build a few gaurdsman units, get a pair of comissars and rush the enemy, very nasty attack. Assault marines are best early game against unupgraded guardsmen, they'll break easily (unless you opponent is following the previously mentioned tactics.) The way I'd play a Space Marine game is like this:
Early game, don't bother with a Force commander unless your fighting a complete idiot. Get some scouts and grab you LP's, and asap get a tactical squad and head right to the enemy base. This works best 2v2, your ally feints with some cheap units against the other player, and brings his best unit(s) with you. This could win you the game right here.
After you hit tier 2, you are the strongest side out there. While everyone else is replacing their basic units, your upgraded marines are an even match, and you only pay for the upgrades! Now is another good time to attack. Take advantage of the chaplain and apothacary healing auras.
Vehicle wise, are whirlwinds available now? These are great units. With 5000HP they are very tough, and do suprising amounts of AoE damage.
Tier 3/4: If for some reason you've not crushed the enemy, you should be well in charge. Predator Anhilators will cope well with any vehicular threat. Assualt Terminators, add apothcaries and heroes into the squads, load into orbital relay and dilute to taste.
I should note that Space Marine upgrades are exceptionally buggy. Powerfist upgrades will reduce your damage, not increase it. The power sword upgrade divides the Force commanders damage by TEN! Plasma pistol upgrade raises some units dps, reduces others. Not worth it. The majority of a land raiders damage comes from it Heavy bolter, not the lascannons.
Heavy Bolters have longer range, are cheaper, and do the same or MORE damage to all unit types compared with plasma guns. They do more damage to some building types than missle launchers. In fact, if i remember correctly, gaurdsman knives do more damage to HQ's than missle launchers. The left lascannon on the baneblade does high building damage, and the one on the right does higher infatry damage.
I could go on all day....
