What Type Of Server To Handle

I have a client that asked me to educate myself about web hosting and make a recommendation to him about where he should be. He currently has a shared hosting server at Network Solutions and finds unexplained slow downs and disk corruption reports in his forums DB unacceptable.

I'm glad I found this site-lots of good info but nothing like throwing up some stats and seeing what people recommend. The client told me he wanted to move to a dedicated server but I'm thinking a VPS might do the trick. Especially if upgraded with dedicated Core as well as RAM such as wiredtree is offering.

Looking for a managed, Unix based server that in a typical month serves
100k unique visitors
230k page views
500Gb of downloads

But needs to be easily upgradeable to handle his expected traffic levels in the next year of monthly visits in the order of:
250k unique visitors
600k page views
1.1Tb of throughput
As far as features:

*Currently they use about 15 gigs of disk space. Some of that is inefficient disk management but the bulk is them supporting previous software releases.

*needs to be fully managed

*US datacenter with all the features you guys would expect to have as far as backbone access, security, power backups, etc..

*Backups by provider. Let's say 5 gigs worth since the old software versions don't really need to be backed up.(I'll recommend his own backups as well)

*Either plesk or cpanel

*15 minute hardware SLA is what the client is asking for but i'd like to present some comparisons to 1 hour SLA companies to see how much he'd save.

And finally, i tried to search for the answer to this but the keywords kept bringing up lots of hits without good info. The client sells software so the bandwidth needed is pretty consistent until they release a new version. Then it skyrockets to the point they may have 1500 people trying to download a 50Meg file simultaneously. What is the right way to handle that? Use a CDN or negotiate with the hosting provider to provide burstable bandwidth as needed. As a side note while looking at many offerings I was most surprised that bandwidth seems to sold in large chunks with overage costs hidden.