Here is my 2 cents.
You should be able to PLAY just fine at 1.5 or even lower. However, I'd recommend at least 3 (DLC, loading and so forth, the more the merrier) - also pay attention to your upload speeds as they do matter.
Other factors as mention are your carriers level of service. If you have high latency, packet loss and all that other jazz it'll all bring down your gaming experience. I don't care if you have 30/30, you'll hate it with packet loss.
When going slim on bandwidth, you really limit your ability to do anything else. You can perform QoS but IMO it is not worth it for consumer issues.
1. QoS values are nuked when they hit the internet. QoS would only work internally which could help....
2. Most consumer devices don't do QoS or they don't do it very well. Especially if it is inspecting, marking and classifying a bunch of internal traffic.
3. QoS needs to be end to end or do it based of IP address if you're not going to use markings. Which now that I think about it, the PS3 PROBABLY does not have any configurable markings?
Long story short, you don't need much bandwidth. However, the carrier needs to be solid and there really can't be much else using the internet connection.
My '03 Sold.