I have an '09 SERG and it has only front speakers.
I did the first upgrade about four years ago to J&M 7.25" fronts for I think around $215.00. They made a huge improvement in sound quality and volume are cruising speed. But still lacked a bit when it came to the bass setting, I had to ride with about half on the bass setting or they would distort somewhat.
Then last year I went to the J&M trailer at Rapid City HD during Sturgis week. I bought a stereo front only amp for the two 7.25" speakers. I paid just over $300 for their kit. It had all the correct wiring and brackets to bolt right in. I would have never thought to mount it the way they did with the kit brackets.
That amp with the 7.25" speakers is the bomb. I'm sure there are many other combos out there as Puzzled has stated, but since I already had the J&M fronts, the amp was a no brainer.
I can now listen with more "hearable" bass while riding at freeway speeds. My BSR has a lot better sound quality too. She can make out the words rather than just hearing a beat and mumbling.
I think you could be happy with just fronts and an amp. Would I like to have bag lid speakers and lowers with speakers, probably, but I am very happy with my minimalist approach.
One thing to keep in mind is that the impedance from different head units are not all the same. I think in '05 or '06 they went to 2Ω speakers. I think most car units are 4Ω, so just make sure that you buy the appropriate speaker for the head unit. I know the J&M offers their speakers in multiple ohm options.