Don't get discouraged. First get your priorities right.
Do you want your system to be a good music system that plays movies now and again?
Or conversely do you mainly want to watch movies and or play games and listen to music now and again?
If it is the latter go and buy the best two channel speakers your budget will allow after careful auditioning. If they have a reasonable bass response the movies will sound fine. Shake the floor no. You can add the rest as funds permit.
If the answer is the former your budget will only allow a compromised multi channel system.
The reason that the subs shake the floor is that the Dolby system has a built in 20db boost for effects like explosions. The speakers have to be tough enough to not self destruct. This 20db boost is only present at the subwoofer output.
Now I will make a surprising admission. I hate subs. I listen to mainly music and watch operas rather than movies. Subs have very poor bass articulation, very evident on good pipe organ recordings were rapid pedal passages are a blur.
The problem with subs is that they are a slight of hand, and only seem to defy the laws of nature. Honestly they really don't and can't. Usually the trick is to build a box that is too small for the driver. Without extensive bass Eq built into the amp, they actually would not go that low. The penalty is huge distortion, especially if the small enclosure is ported. Ported enclosures respond poorly to having their bass extended electronically. Big ported subs are another matter. The worst solutions tend to be the coupled cavity subs especially the third and fourth order variety, were the bass phase shifts, time smears and distortion are horrendous.
My solution has been unique and I know of no commercial design like it. I have built large transmission line speakers that extend down to the 20 Hz range. F3 IS 27 Hz. There is lots of output to 20 Hz, and excellent room coupling. I have added the LFE channel to the system electronically. I know of no subs that have anywhere near the impact of these. They also have an accurate non boomy articulate bass. But then I did not try to bend the physical laws which govern us. I worked with them.
Here is the picture.
http://mdcarter.smugmug.com/gallery/2424008#127077317
There are other features not found in commercial designs also, such as active feed forward correction of the step response errors.
So I would go with what suits your life style and budget. There is time to add revise and extend over the years.
One last thought. Two good channels beats five or seven lousy channels hands down.