I have been planning to upgrade my system within this month or the next. I'm trying to keep everything within a budget of $300.
I've decided it was crucial to buy a new case (I currently have a smaller sized case that gets very hot with my current setup). I also want to upgrade my graphics card from a nivdea BFG 9800 GT 512mb.
This might be a stupid question, but is there some limitations to your current system you would like to get rid of to justify upgrading?
Moreover, could you confirm all your current specs, because the only "550" Phenom I can find is a dual core, not a quad as you mentioned, and it's supposed to be 3.1GHz, not 2.21GHz. Did you mean the Phenom 9550 @2.2GHz? Have you considered overclocking as a cheap way to upgrade? From what I read quickly, you can probably hit 2.6-2.7GHz with proper cooling.
How much RAM do you currently have? Just switching from DDR2 to DDR3 will probably not be worth the investment.
A 9800GT is still a "decent" video card, any games causing you problems? What resolution are you playing at?
With all regards, That's not a great budget to consider all your desired upgrades for now.. May be you can uphold on a few things and go for upgrading them later.. Now the few things that you can surely upgrade are-
1. You need a case with good airflow urgently and i think the NZXT Tempest EVO fits the bill nicely..
2. In case your motherboard supports phenom II X4 series then its wise to go for something like the Phenom II X4 955 BE..
3. There is no need to go for DDR3 right now as the current crop of AMD processors barely show any significant advantage with the extra bandwidth..
4. I'd suggest you not to consider the 4850 as of now.. If you can add a little more cash then may be you can go for the HD 5770.. They'd perform on par with the old 4870 while bringing the dx 11 and eyefinity advantages to the table..