Like Fuzz, I think there's more to be squeezed out of the 4.7L motor without significantly compromising any reliability or driving characteristics. 50-100hp is probably likely on a reasonable budget. Personally, I'd spend about $3-4k to get the HP and torque figures in the 300HP / 375lb/ft range.
The biggest question is what's the path to getting more power in a cost effective manner for the 4.7L. We already know these levels of performance exist ...
Cost range - Product Mod - Power - Cost
$0 - 1k - DT header - 22hp / 10lb-ft - $650
$1k - 5k - Unknown - 50-100hp (maybe?) - NA
$5k - 10k - TRD SC (disco'd) - 125hp - $7k (G'luck!)
$10k - and up - SleeTurbo/Bullet SC - +150hp - ~$15k
- 5.7L conversion - ~150hp - +$15k
Are the TRD SC's even possible to find? I'm willing to spend 6-8k for the 125-150hp but can't find a TRD SC to save my life.