In general, you don't host with a provider solely for speed. A company like Rackspace with dedicated servers (I know they don't do much colocation so I'm assuming you're using a dedicated offering) is giving you:
- 24/7 monitoring, depending on the level of the offering you purchase
- SLAs for uptime, etc
- Flat-rate hardware (i.e. you pay a month-to-month regardless of necessity of replacements, etc)
- Powerful existing infrastructure (load balancers, firewalls, etc)
- Multiple internet backbone providers (in the case of Rackspace's largest datacenter I believe it is seven)
- Minimum N+1 redundant power and cooling systems
- Physical security
- Expert support availability
- etc, etc
You don't host with a provider like Rackspace for the purpose of raw connection speed. That's like asking how many RPMs you'd need to hit in your Toyota in order to match a Ferrari. It's certainly a metric, but it's meaningless without context.
Now, you may be able to provide some or all of the above yourself (and there are many unmentioned factors), but the real question when looking at self-hosting, colocation, dedicated offsite servers, "cloud" hosting, etc, is reliability, performance, and efficiency per unit cost, and what is an acceptable level of each. There are no easy answers.
Ultimately, though, to answer your question, on a pure speed basis you're at the very least going to need a dedicated line à la T1, or some other symmetric connection. Your typical home-style connection, and indeed many "business" hookups, just don't have the outgoing capacity, especially not in a reliable, non-bursty manner.