Phenom II x3 720 BE worth the upgrade to a x6?

Lol, then why do you have a 5770 if you hardly game?

Anyway, no one can give you an answer to your question unless you tell us what you use your computer for, specifically. For all we know, the computer could be overpowered for the tasks as is. Give us details and I will give you specifics.

Generally though, in most cases, keep your current system and change nothing. It will not be worth the cost now. It is also likely not worth it for your uses at all. 4GB is still the "sweet spot" for memory, if just barely. 3-cores is the most cores that most current games use. Next year, AMD will release their new Zambezi CPUs and you may regret spending $300 on a 6-core if you can get an 8-core for less. At the very least, Zambezi will reduce the cost of current CPUs.
I say wait for Zambezi. Then upgrade your CPU or, if a socket change is needed, your CPU, motherboard and memory.

One thing to know is that it is always worth waiting in the computer industry. But really, you can't wait forever.

I got a 5770 cause I was gaming more when I built it(when I do game I love having the graphics power as I tend to place games on the highest settings). Mostly I use it for running linux and doing development, running VMs and transcoding. Now the reason I am thinking about this as I kinda pointed out is because I have actually a few times now ran out of ram.

I can definitely see that with VMs and transcoding. Are you running DDR2 or DDR3 memory?

Since you are doing VM, you will definitely need the extra RAM specially if you actually see the 4GB being used up.

Bt the way, what is your motherboard? If by chance it have the unlock core feature, you have the opportunity to get an extra core for free.
And being a black edition, the X3 720BE is very easy to overclock too so you can mildly boost your CPU performance at no cost. (For higher overclock you need an aftermarket CPU cooler ).

Try those first before you consider spending for an X6... although to be honest, the X6 is tempting for me too ( I have an unlocked 720BE myself ) but the good advice from the people here kept me from wasting my money...  

I have DDR3 on a AM3 mobo I believe its a 790X.

Well, the 790X is not the complete motherboard model. But that clearly tells that it is a 790X chipset implying that you got a good motherboard for both unlocking and overclocking the X3 720BE. If you haven't tried tinkering with the BIOS, it is not difficult at all. But read up your manual first as it will have specific guides on how to manage it.

Just come back here and let us know if you need more help on that department. Or you can open up a new thread how to unlock and overclock your CPU with your motherboard.

Actually I did try unlocking the other core when I first got it... It didn't work.

If you are willing to unlock/overclock, the Phenom II x6 1055t is a worthwhile $200. 2.8GHz stock, no unlocked multiplier, but from what I have read, overclocks just as well as the 1090t (so far anyway).
Just remember that next year amd comes out with Zambezi. There is a possibility that it will be compatible with your chipset and socket, although no one knows at this point.

Personally I would just pick up more ram, than after installing that if your system still feels sluggish get an X6. I just don't think you really need it or that it may not show much of an improvement for what your doing.

Too bad. But that does not stop you from overclocking.

Anyway, the first time I attempted to unlock my X3 720, it also fail but it is because I fail to do one more step required in the BIOS. But after more attempt, voila! an X4 stable up to 3.6GHz.
What is your motherboard model and how did you attempted the unlock?

So you don't game then, i mean on the whole. Do you multi-task a lot and use multi-threaded applications alot. In that case, probably an x6 would be good. But for games you won't see an increase in performance. If you can unlock the 4th core on your phenom ii 720 that will be all you need for gaming if there are those few games that you play that make use of four cores or 3 cores. I don't think you will see the slightest FPS increase at all from a quad core to a 6 core just playing one game with no applications running in the background. I think unlocking the 4th cores should be soo soo easy. It must be, it either works or doesn't. That's because some x3's actually have the 4th core mulfunctioned so it won't enable if you try to get it to work, some do because the 4th core is just disabled but isn't manufacture malfunctioned. I think all you do is just enable advanced core callibration, and then select unleash mode i think if you have an asus board, and then select core 2, core 3, and core 4 to work. Core 1 is not there because you obviously can't disable the first only working core in your computer. That's really all that needs to be done and if it isn't successful as i said the forth core is non-deliberately manufacture malfunctioned and sold as a 3 core.