If you install the drive internally, you will of course no longer have access to OS X, though the drive should startup. Badly though since the drivers for your Mac's hardware won't be on the drive. I don't recall if Windows 7 will boot from an external drive. I believe Windows itself doesn't allow it and will refuse to startup from an external if you try.
Do note that this is a lot of work.
Since I'm pretty certain you cannot boot Win 7 from an external drive, you have to get it on your internal. To move the install you have so you don't have to install Win 7 and all of your installed software from scratch, you need a third party software package.
1) Purchase Winclone. Follow their instructions for preparing your Win 7 install from a PC to a BootCamp partition. This does involve having the Windows drive in a working PC.
2) Use Disk Utility to create a FAT32 partition at the size you want Win 7 to occupy. You can do this without reformatting the drive. Make sure you have a complete, restorable backup of your Mac before proceeding in case something goes wrong, or you goof up. Launch Disk Utility and click on the physical drive name of the Mac's drive. Click the Partition tab. Drag the bottom right corner of the Mac's current partition up to create an unused area on the drive. You will only be able to drag it up as far as there is no live data, which will be shown in blue. Make sure not to crowd your OS X partition and starve it for space. Click the + button. A new partition will fill the empty space you made. By default, it will be Mac OS Extended. Change it to MS-DOS, which will be a FAT32 partition. Click Apply. Don't worry that it's not NTFS. When you restore your Win 7 disk image, it will automatically become the same file system as the source.
3) There is the issue of partition size. If a cloned Windows drive takes up 500 GB on its source drive, that's how much room it will take up on the target drive, no matter how big of a partition you made for it. Say you only want Windows to only take up 100 GB of space. When you restore the clone, it will force the partition size out to 500 GB, because that's the amount of space it took up originally. This is not an issue with Winclone. It's just what Windows does when being cloned. Using something like Symantec's Ghost to restore a Windows disk image on a PC will do the same thing. It will only become smaller if the overall size of the target drive itself is smaller and it can't take up its original amount of space. To force the target partition size down, you have to shrink the Winclone image. I've done this myself, and it does work.
4) Once you finally get the Windows clone onto your Mac's internal drive, follow only the BootCamp instructions to get the Win 7 drivers for your Mac ready. Boot into Win 7 and install the initial Win 7 drivers for your Mac.
5) Once that's done, the graphics will be rough at this point. You'll now have an Apple Software Update menu item in Windows. Run that to download and install any other BootCamp drivers it lists. When you restart after that, the desktop should then be correct.
