Hard drives affect the following:
- How fast a program will load and how fast it can load things.
- How smooth streaming data is. This usually affects video games since movies have a data rate that's much slower than any hard drive.
- How fast you can write something. This usually only happens with video editing programs.
For an SSD, I usually keep my main programs on it and everything else on the HDD.
4GB is good. If you really want, you can upgrade to 8GB. But no more than 8GB. No games are built in 64-bit and by default, Windows 64-bit only allows 2GB for 32-bit programs anyway (most games are good about keeping within this limit). If you really want to, you can upgrade. The only thing you might see is more response when you have a lot programs open at once and you switch back and forth between them.