At this stage, the best upgrade for you would be to simply replace the PSU and CPU cooler, add an additional 12GB of RAM, and overclocking the CPU. Here are the reasons why I'm recommending this particular upgrade:
1) The PSU really cannot deliver more than 575W in realistic internal PSU operating temperatures of about 50°C. In fact, OCZ cheats on the rating of its mid-range PSUs by using a lower-than-realistic operating temperature of 40°C. Replace it with a good-quality 850W+ 80-Plus Gold certified PSU such as a Corsair AX850.
2) The CPU cooler is too small for a hot-running CPU such as the first-generation i7 CPUs, especially when overclocking. In fact, its single 92mm fan isn't anywhere near large enough to handle all that heat. Replace it with something like a Cooler Master Hyper 212 EVO, which is designed to be used with 120mm fan(s).
3) You will need to replace all six sticks of your current RAM with one 6 x 4GB kit. 12GB isn't quite enough for use with CS5.5; in fact, there is very little difference in performance between 12GB and 4GB with CS5.5 - and there is a much greater performance difference between 12GB and 16GB than between 4GB and 12GB. So, with RAM going for cheap nowadays, go with 24GB total for this LGA 1366/X58 platform.
4) Overclocking the CPU to at least 3.8GHz will help that system's performance - often noticeably (when it comes to Premiere Pro CS5.5). But make sure you upgrade everything else mentioned above before you overclock the CPU.
Now, you might ask "What about a CPU/motherboard upgrade?" Well, at current prices, you would have already busted that $760 budget (including most of the required hardware upgrades, but with 16GB of RAM instead of 24GB and adding an i7-2600K CPU and a Z68 motherboard) and still would only give you a sideways-grade in performance for all that money (unless, of course, you are willing to heavily overclock the new CPU). And forget about an upgrade to an LGA 2011 platform right now because the least expensive motherboard/CPU combo costs almost $1,000 by itself, plus you still need to make all of the aforementioned hardware upgrades plus an LGA 2011-compatible CPU cooler (and after all that, you'd still be saddled by a less-than-balanced disk system and an older, slower GPU).
I second Randall's well worded arguments.
I will also vote for Randall's changes. If you then set it up and tune it properly you will have a very good AVCHD editing workstation. Take a look at the #6, #14 and #15 entries on our PPBM5 page have been able to do. You will not get it there but you can see what quad core i7-9xx's have been able to do under ideal conditions.
I forgot to mention that if you are still using the stock 120mm fans that came with that Thermaltake Mozart TX case, you can upgrade those fans with higher-quality (and better-performing) 120mm fans such as the Scythe Gentle Typhoon series fans. And despite the case's mere 360mm (14.1-inch) depth, the airflow is actually quite good in that case because the hard disks and optical drive are located way up - completely out of the way of the motherboard's expansion slots. This makes upgrading the stock fan(s) with better ones fairly easy.
tanks for the reply
I accept your advice
Just noticed you did not mention anything on hard drive
Those thist means that i'm in a good situation on hard drives or it's becuse my Budget?
It's more because of your budget:
Right now, the prices of hard drives are more than double what they were four months ago due in large part to the floods in Thailand that severely affected hard drive production (and ultimately availability). A 1TB 7200 RPM hard drive used to cost $60 in October of last year - but now costs $150 to more than $200 each (last time I looked at Newegg a couple of days ago, the 1TB WD Black WD1002FAEX cost a whopping $247 each, shipped). According to current price trends, it will take almost another year for production to return to pre-flood levels and another few months after that for prices to fall significantly from their current inflated levels.
As such, after accounting for the price of the replacement parts that I recommended in my first reply, that barely leaves you with enough money to buy even two hard drives. You will have to increase your total upgrade budget to more than $1,000 to accommodate even four hard drives in addition to the aforementioned recommended upgrade/replacement parts - and even then, you may run into the restrictions of a given reseller: Some resellers and most retailers are now limiting hard drive purchases to only one drive per customer per day or visit. That makes it more difficult to receive drives that work together properly in RAID.