For that price, you should be able to get at least 1Gb of RAM (look for 2Gb for Vista), a dual core CPU (Core 2 Duo is probably your bets bet, although Turion X2 is pretty good as well) and whatever else you want.
It's a good price range for a non-gaming laptop.
4Mb cache means more cache for the CPU, so it should be faster. Realistically, you won't see much of a difference.
Core 2 Duo replaced Core Duo, it's faster, uses less power and supports more features, like 64-bit.
Hope this helps a little.