What type of amplifier do i need?

I highly recommend that you not choose your amplifier/receiver based upon powering two bookshelf speakers. Any amp that accepts 6 ohm load and puts out 35-50W RMS will take care of the speakers. The max wattage rating is to subjective to use as a guideline. With a two channel amp, the max power requirements must be at least 100W (from the outlet) in order to back up an RMS claim of 40W per channel (x2). I recommend you choose your amp/receiver based upon everything that you want/might want to use it for. Just music? Just those bookshelf speakers? A stereo (two channel) receiver/amp will do just fine. If you want to use it for tv or movies, an av receiver (audio video) is what you'll need. Consider what types of inputs you'll need. Record player? it needs a "phono" input. Dolby Digital 5.1 surround? it needs a digital audio input. Although, 6ohm is a little heavier than standard, which is 8ohm, so you'll want to pay attention to that detail. Happy shopping.

Keep in mind that a stand alone power amplifier are mainly for the higher end of the audio. They are also more expensive. I would recommend you look at a receiver, either a stereo or a multi channel receiver. Most receivers can handle the 4 to 8 OHM load resistance of the speaker. Pioneer and Onkyo are the top brand for reliability. Power rating are only a guideline. Hope this will help you out.

Well, the other part of your question is, what kind of applications do you want the amp to be able to do.

An audio receiver, which includes an amplifier within it, lets you connect other devices, such as a disc player, and so on.

For $100, you could get a Sherwood RX-4109 on Amaz*n. For ten bucks less, you could get a Technical Pro 72-RX35U 800-Watts TP Pro Digital Home Receiver with iPod Input.

Both of those would be quite compatible with your speakers.

IF you are just looking for a simple two channel amp look at the AudioSource Amp-100.