I'm a newbie looking to get a DE (maybe Gillette Superspeed). How do I know which blades to get? Do they make new blades that fit perfectly in these razors from the forties?
Any new DE blades you get will fit any DE razors. You might get some wierdness in an Gillette Old type as it was designed for a thicker blade, but they will still work fine.
Get a sampler for the different brands, and try as many as you can to see which works for your face.
I'm sure someone can point out a weird scenario where there is some obscure DE razor that does not fit this bill... but if you stick to the usual suspects (old gillettes, Merkur, EJ, etc...) you will be fine.
There are at least 12 pages of reviews of specific brands here http://badgerandblade.com/vb/forumdisplay.php?f=50
and probably hundreds of others out there in the market. Check out westcoastshaving or bullgoose, they offer some nice sample packs of blades for you to try out.
As others have mentioned here you should try and get a sampler pack from Westcoastshaving, Bullgoose shaving supplies. Both gentlemen are great to deal with and have great customer service. Once you have used a few samples look at the review section and see how they measure up against you personal use.