The FX-57 as clement said is a single core while the only Athlon 64 4400+ for your mobo which is a S939 was a Athlon X2 4400+ so you currently have a dual core CPU.Going to a FX-57 would in the end downgrade your gaming capabilities because most games will use at least two cores, Crysis for instance will offload the sound processing to a second CPU core if you have onboard sound.
If you want to do a simple upgrade for gamning purposes, then since your mobo does have PCIe x16 you could get say a ATI HD4850 thats about $99 or a nVidia equivalent like a GTX260. That would benefit you and you could even use that same GPU for your i7 build later on and get a second one of the same and do SLI/CFX which would match a Core i7 rig nicely.
How much were you planning to spend on that 2.8ghz FX57? The price of the ones that I have seen online would buy you a new AM3 motherboard, DDR3 and a new quad core cpu.
As mentioned by clement, it would be about the same speed as your 2.2ghz dual core 4400. Older games that use only a single core, would run faster but newer games would be the same or slower.
Faster socket 939 processors have become so expensive that it isn't worth upgrading. A dual core FX60 runs at 2.6ghz and is selling for $250 on ebay. You can buy a dual core AM2 6000 which runs at 3.0ghz for $60 at newegg.
The long and costly upgrade has started, ive purchased an asus maximus formula 3 and will plunge for an i7 860 very soon then its just ram, video card, psu, case, hdd etc etc.
looking at asus 5870 video and corsair dominator 6gb 1600cl9t think they will work well together. thanks for heads up guys and if anybody has advice on new system spec Im all ears.