Unfortunately, Cisco devices often have very different QoS features. In your named devices, the 3750 and 3560 QoS are alike, but differ from both the 6509 and the 2800, which also differ between each other. The 2800, being a software based router, has about the most "powerful" QoS feature set (and is the only device, I believe, that could configure a QoS policy suggested by Georg [also that policy might not work quite as Georg describes, and I would recommend against it for other reasons, including a 2800's QoS works differently, using class-default FQ, whether using a pre or post HQF IOS version]). The 6509's QoS features can also vary per line card. Further, the 6509, I recall, offers MicroFlow policing, which the other devices do not.
You also need to further define your "bandwidth control". For example, to the Internet, or from the Internet, or both? Do you want to cap bandwidth, guarantee a minimum bandwidth, or both? When capping bandwidth, do you mean to just discard overrate traffic, or queue it? Also when capping bandwidth, do you intend to allow any bursting?
Again, depending on what you can accomplish, much depends on the device. If your needs are complex, especially for managing from the Internet traffic (so just one IP or group of IPs, don't congest your Internet link), you may need to look into using a 3rd party traffic management device.
If your 2800 is the device that actually connects to the Internet, for to the Internet traffic, perhaps a policy such as (using a "later" IOS):
policy-map toInternet
class class-default
bandwidth 100 percent
fair-queue
Might be sufficient. If the port speed is faster than 50Mbps, but your ISP caps you at 50Mbps, then instead of applying the above to your interface, you apply the following policy (keeping the above policy too) to your interface:
policy-map toInternetShaped
class class-default
bandwidth 100 percent
shape average 50000000 !NB: you may need to set this about 15% lower to allow for average L2 overhead
service-policy toInternet
Managing inbound Internet bandwidth, is very problematic on Cisco devices. There are also configuration limits, such as the number of classes allowed within a policy-map. I.e. if you have lots of IPs and/or groups of IPs to manage, you might bump into those limits. 6509s, with the sup720s (I recall) also have limits that can be bumped into if trying to rate limit all its possible edge ports.
Lastly, again if your 2800 is the Internet connected device, if may struggle with 50Mbps, duplex, depending on what model 2800 it is.
