When editing HD video, and I assume you mean AVCHD video, the general rule is the more powerful CPU the better, see
here for a previous discussion thread, most of which is still relevant.
The i5 660 is rather marginal for AVCHD editing, it would work but with sluggish response. A better CPU would be an i7 860/870/875K/880, which would work in the same motherboard. Fluid video editing of AVCHD and encoding is all about processing power (the need to decode every field or frame before anything can be done with it).
Contrary to popular belief an advanced graphics card is
not required for general video editing as the majority of graphics processing is only 2D, although using the onboard Intel HD graphics is rather optimistic. Advanced graphics cards are only generally required for some FX such as some transitions and special 3D generated effects. You should aim to get at least 512MB of graphics memory. Some video editors use nVidia CUDA for video encoding, but will fall back to CPU encoding when not present. Only the very latest version of Sony Vegas can make use of nVidia CUDA for encoding. Other than that all processing is done by the CPU.
You can add a discrete graphics card to the motherboard, and it would be best to add a lower-end card with 512MB of graphics memory. If you want to use multiple monitors, or a monitor of very high resolution, it would be best to use a card with 1GB of memory. The lower-end cards dissipate much less power than the mid range cards and do not require an additional power connector. However, you must ensure that the PSU can support the additional load. The ATI cards draw significantly less power than the nVidia equivalents, but do not support CUDA. I would suggest a Radeon HD 4650, or an nVidia GT220 as a minimum.
As an aside, the new Sandy Bridge processor architecture, released in January and February next year, will have much more powerful onboard graphics, equivalent to the lower-end discrete cards, will support Open CL, and provide very powerful hardware video decoding
and encoding. The CPU's will also be more powerful by a few percent. It will be interesting to see how these work with editing applications.
An SSD will significantly reduce Windows boot time, and speed up the loading of applications. Other than that, an SSD will not offer any significant benefit for video editing. The Seagate HDD is more than adequate for your purposes. Currently, SSD's are really only justified in high performance workstations. At a lower cost than SSD, you could use Raptors, but it would not bring any significant improvement to editing, and would be relatively poor value. Similarly using HDD's in a RAID setup would not really help, and furthermore a RAID 0 array is not wise, because of the doubling of failure probability. A more powerful processor has a higher priority.
4GB of 1333MHz RAM is fine, and since most (all?)
consumer video applications are 32-bit, the amount of memory they can use is limited. You will find that not all of your RAM will be used unless you are heavily multitasking - which you should not be when video editing because you want all the CPU resource for the edit processes.
The GA-H55M-S2H is a budget motherboard but will do what you want it to do. The Intel ICH does not support RAID, but I assume that this will not be important to you. Again though you could use a low cost add-in RAID card at a later date if this is required.
In summary, if the system already exists then it can be upgraded to give a good AVCHD editing experience. Start by upgrading the graphics card, and then if the editing performance does not match expectations replace the processor with an i7 8XX. However, check the capacity of the PSU before upgrading. If the system does not currently exist then choose a system as discussed in the forum thread
here.
Rather than edit raw AVCHD an alternative is to convert it a less compressed format for editing - there is lots of discussion about this in the forum.
