Buy anything and everything in high capacity that you can as far as handguns. You should also stock up on high capacity magazines for your existing inventory of handguns. In the rifle category, you might want to pick up an AR, an AK or an SKS before they disappear from the landscape. If you can find a decent SKS surplus, take your time cleaning all the cosmoline off and then visit TAPCO online or order their catalog and outfit your rifle with an adjustable pistol-grip stock to make it more functional with lights, optics. etc. I used to think we'd have a year or so after January '09 before the DemTrain built up a head of steam. But lately I've become more convinced that we'll see a flood of executive orders on January 21st and we'll be in a fight to protect our God given rights immediately.
Personally, I've been on a gun-a-month plan. It'll take some time to replace the money I'm going through in savings, but at least I've got the high-cap Para .45 I've been wanting and a few bucks went to the NRA via Para's donation. I'm adding a Springfield EMP (never thought I'd own a 9mm), and I'll probably add one of Ruger's new SR9's unless they tell me a SHOT in a couple of weeks that I'll be seeing the SR40 soon. Once I get the high cap guns added to my gun safe, I'll spend early 2009 adding some Dillon Precision reloading equipment to my gun shed. I figure I'll get a 1050 and set it up full time for .45 and probably another to split time between .38 and .40. If there's any money left and I still have a wife, I'll add a 1050 for .223 and another 1050 or 550 for 7.62/30.06/.308. As far as rifles, I'll be adding an Armalite AR-10 and a Springfield SOCOM to the gun collection for July and August.
This probably isn't the time for the esoteric/eclectic calibers. You'll want some reliable, easily obtainable calibers. I'd also recommend that you read every last word that Jeff Cooper ever wrote and start getting your mind right to stand with your fellow gun-owners against the coming storm.