MaximumASP
I don't know what you consider affordable however I use MaximumASP's dedicated server B200 plan ($299/month) with the optional SQL Server 2000 Workgroup license ($75/month).
I run DNN 3.1.1 (the most stable version that i have come across) with mulitiple (10) portals.
The speed off my DNN portals is amazing - even under heavy traffic loads. The server has dual xeon + a gig of ram.
MaximumASP will not fail you. I'm sure the folks at PowerDNN feel the same way, but I'm hooked on how fast MaximumASP resolves issues. They also have a MINT datacentre with sophisticated intrusion detection systems.
Only my satisfaction with MaximumASP, has prevented me from investigating the services of PowerDNN. If they had comparable dedicated server plans similar to the business series or even enterprise range of dedicated servers that maximumasp offers, perhaps I would consider them.
I, nor my clients can afford to be left alone when things go wrong. I feel 100% safe and secure with MaximumASP based on my experiences with them.
Dotnetnuke itself uses MaximumASP.