Can't decide SSD and HDD or 2 HDD raid0

Are you looking to decrease your boot/application startup time or continuous read/write speed?
RAID 0 is good for when you copy large files (i.e. due to stripping) but won't (significantly) decrease load times (as the head movement is still slow).
SSD has a (figurative) 0ms seek time. So loading applications (which are many small file payloads) is faster.

And 4GB RAM should be fine for current gen games. Although running BF3, my system uses 4GB total (out of 8)

I just want what Evers good for gaming not to bothered about bootup

Go for the SSD and another 4GB stick of RAM. Putting in a 8GB stick is going to cause issues.
The SSD is certainly going to boost up game loading times and will help a hell of a lot especially when you game online.
It's a lot easier to just plug and play then go down to the RAID configurations.

SSD and HDD. Raid 0 in many cases wont give you ANY performance increse.
Also raid 0 in hdds has a quite huge chance of failiure. You might think that 1 year is long till it will fail, but if you get lights out, or anything like that, chances of failure go up like hell.

Also once you have most of the start up progrmas on an SSD, the rest goes quite smooth.