I'm natively editing a 1080i avchd project. The project will play smoothly for 20-30 sec, then the video slows down so badly, I cannot judge the piece as a whole. There are no effects on the clips either. I know avchd is an intense codec for a cpu to process, however, I've edited avchd natively before and don't recall experiencing this issue.
Windows 7 64bit
quad core 2.2 GHZ
All media on 1TB G-Drive connected via esata to sata port on mobo.
The only thing I can think of is maybe there's something going on with the G-drive?
Couple of other things to consider: indexing, compression, sidebar, aero, anti-virus or anti-anything, other background processes, active services, auto-updates, QT, etc.
1-you may not have enough ram
2-does that gt 240 have hardware mpe?
My CS5/AVCHD 1st Impressions http://forums.adobe.com/thread/652694?tstart=0 includes a link to the computer I built... since the GTX 285 is no longer sold, I would now go with a GTX 460 or whatever is the current "best bang for the buck" - AND I would buy 4Gig-by-3Sticks memory to be able to expand from 12Gig to 24Gig if needed
For my home hobbyist, family movies (which means that I am not trying to recreate Star Wars with video effects or many layers) AVCHD editing is "as smooth as spreading warm butter on hot toast" (also the MP4 video from wife's Flip camera)
My 3 hard drives are configured as... (WD = Western Digital)
1 - 320G WD Win7 64bit Pro and all programs
2 - 320G WD Win7 swap file and video projects
3 - 1T WD all video files... read and write
For a list of supported nVidia CUDA cards go to the FAQ
For "some" others, with at least 1Gig of video ram, the nVidia Hack http://forums.adobe.com/thread/629557?tstart=0 - which is a simple entry in a "supported cards" file