Building a new PC

Hi all,

I'm trying to build a non-gaming (maybe light gaming) PC which is going to be used for at least 3-4 years. I watch a lot of online videos, browse the internet and download.

I have a Samsung SyncMaster 940NW monitor (1440x900 i think) and will be using this same monitor for my new build.

So far I have decided on 3 things:
1) my motherboard is going to be Asus P7P55D-E EVO (or Asus P7P55D-E Pro, cuz it's cheaper, i might need the extra euros for an SSD)

2) My processor: Intel i7-870 (quad core, 2.93ghz up to 3.6ghz) and,

3) Radeon HD5570/HD5670 (either one of the two video cards)

So, where I'm stuck is whether or not to go for SSD drive or a normal 7200rpm hard drive. I only need maybe 500GB in total. I also know that SSD is much more expensive, but performance wise is very nice to have.

Currently I'm looking at getting either 2x250GB drives or maybe a bunch of smaller capacity SSDs or both?

Another major problem for me is to pick a case. My new motherboard is ATX, I just want my case to look cool (like cooler master scout or smth), have enough space for possible upgrades and i want it to be not so big. I'm confused with sizes and towers after researching it and I'm not sure if new ATX case towers are the way to go, or I can just look at other older options (such as cooler master scout). I need something that is of good quality and can be used for years.

Oh, I forgot to also mention that I want to have maybe 6GB or 4GB of Ram 1333 (or 1600 if that's possible), but have no idea if it's sufficient on a build with Windows 7 home premium 64. The choice of the proper ram is a problem too, because I think I can either do it in 4GB, 8GB or 16GB, but not 6? The slots and the exceptions to what pairs of modules I can use are all confusing a little.

The power supply? Can anyone suggest a good power supply that has no more than 600W on it, since I hope it's sufficient enough for my uses

Please help!