To be honest mate 6 x 9s dont perform that much better on an amp unless they are boxed, because they are very efficiant speakers they dont need the extra power, the sound quality may improve slightly but not a good pound for pound purchase. A better way is to power the front components or if you dont have after market fronts then spend the money on these. If you do decide to get an amp then there is no hard and fast rule which one to buy and the main factor is budget, to get a decent result you will need to spend £100 - £150 on an 2 channel amp, which also would get you a very decent set of front speakers.
Hope this helps
Luke