Cool, the additional details about your current system help to better understand your plan. I can see the need to start looking at a replacement at some point.
As for performance difference between Bootcamp and Parallels, I don't have any experience with Bootcamp. Parallels has been fine for my use over the years as I use my Windows 7 VM infrequently and not for any memory, CPU, or disk intensive work.
https://forum.parallels.com/threads/...rmance.324629/ is on discussion on the topic. I saw a few others with a search.
One thing to consider with Parallels is that you can have concurrent access to your OS X and Windows environments. I have a dual monitor setup, so if I need to run the Windows VM and plan on using it for a while, I just move it to the 2nd monitor and maximize the window. Looks/feels like running Windows 7 natively. Parallels has tons of other cool/useful features. So if having two OS's available at the same time with deep integration between the two is important and beneficial for your needs, then Parallels may be the way to go.
The other cool thing that using Parallels or Fusion will provide is the ability to run different development tools if you have apps or languages that require dev/test in a particular OS. While this is probably not a big obstacle for web development, it could matter if you want to develop for Windows or Linux. In the Windows case, I've found Visual Studio to be excellent tools. Even the free Express or Community versions are full featured and quite capable. Until those can be run natively on a Mac (could happen), you'll need either a dedicated Windows system or a VM.