D90 vs D3100

I have a D3100 currently, with the 18-105 and the excellent 35mm f/1.8.

However I also have an F4 with the even MORE excellent 50mm f/2.8 macro and the AMAZING 80-200 f/2.8...

which don't autofocus with the D3100.

Now I'm a bad photographer at best. At worse, most of you guys would hit me without remorse nor mercy. But I do find the lack of AF on the D3100 a little annoying to use with the lenses I use.

I don't mind losing that extra ISO range nor those extra MP: the truth is I NEVER do any post processing and never had, so there won't be much software zooming, RAW or anything. I just shoot what is, usually using A/S modes. There's enough to learn with this already!

I could get that D90 body on sale at a local store and sell my D3100 on the cheap; what would I really lose in this?

Thanks!