I have a mac pro and do a lot of uploads and downloads of media files, the problem is my ftp client software and my other clients are so fast efficient that i run into this problem where i have no bandwidth left for Opera, so while my downloads are blazing i can't parse a youtube video unless i wait twenty minutes for it to load like i'm on a 28K modem. consequently even browsing is slow and it took me close to fifteen seconds to load this page. Any software that could do this would be appreciated, as would any kernel solutions.
You need a Quality of Service (QoS) solution. This is usually best done at the network hardware level such as a router or switch.
Virtually every router out there actually has some kind of basic QoS service, which is probably all you need. You just put HTTP and HTTPS traffic into the highest priority, and leave the rest alone unless you want to further prioritize things. You can check with whatever router you have, and see if it has QoS, as well as how well it works if it does. If not, then you probably want to look for a router that advertises itself as a gaming router. I might further suggest taking a look at the list of supported routers on the DD-WRT and/or Tomato website(s). These are alternative firmwares for a number of routers which are often a bit better than the official ones for people like you who might need a few basic features of a managed switch, but not enough to justify the expense and time needed to properly configure a managed switch.
Another option, is almost every dedicated FTP client will let you put limits on your upload/download rates. So you could go in and set them to a maximum speed of say 80% of your total bandwidth, ensuring that you have at least something left over for your web browsing.