Hi,
You may not need to buy anything. If you have Final cut then just export your clips as a targa sequence. I have AE 6.5 Pro so this may not work with AE 5 but here it is anyway. Import that targa sequence to AE and set your composition to 1920×1080. You may have to resize your footage to correct the aspect ratio because it will squeeze the image to 1440×1080. When you complete your composite export as a targa as well. Import back into final cut and export as whatever you want. You should be able to keep the quality pretty high and I believe, if need be, output back to HDV. A few warnings. Lots of image data! Hope you have a fast computer cause this will slow it down. Export your targa sequence to it’s own folder! One picture for every frame adds up real quick and your drive and desktop will be a mess instantly. Remember to change FCP’s timeline to accept the targas! If you don’t you’ll pull in a whole bunch of 10sec individual images. You may also, in the interest of time and space, want to export from AE as a quicktime. You’ll lose quality but you have to think of the end product. The only place I know of right now (and I may be wrong) to play HDV back is from the SonyHDV deck or from the camera. Unless your client or whoever will be watching on one of those devices or on some other form of HD player it may not be neccesary to keep it at this high of a quality.
Good Luck.
p.s.
Shake rocks for keys and composites. I find I use AE more for motion graphics or creating elements that will be composited in Shake. If you like this stuff you should check out combustion’s particles. Those are sweet.
If you already have AE and you have a good handle on compositing then get Shake.