my components run off a 4x70watt headunit and I find this more than loud enough.Loud enough to be uncomfortable infact. As for the sub that depends how loud you want it. I went with a 10 inch and its more than adequate inside the car but if you want everyone else outside the car to feel the bass I think a 12 inch would be required.
No offence mate, but the RMS wattage on that stereo is still likely to be about 20w RMS, as opposed to an amp which would comfortably knock out 2-4 times that and still be in the 'budget' range.
If you want SQL, you've got to go down the amp route - and a 3/4channel jobbie would run a sub and fronts for not a HUGE amount more than a 1/2 channel.
Of course, if budget is a factor and you don't want BIG sound then you don't NEED an amp - it comes down to exactly what you want, and how much you're willing to spend.
That's my tuppence worth on a Friday morning. Really should do some work.