I am currently using a Dell Inspiron 546 PC that has an AM2+ motherboard. I have done everything I can to upgrade this PC as best I can.
It now has a 550w power supply, Radeon 5670, Phenom II X4 945 (Deneb), 8g DDR2 800 ram.
Even with those upgrades this PC has always been a bit sluggish.
My question is would I notice an improvement upgrading to an AM3+ motherboard and DDR3 ram keeping the same power supply, CPU and video card?
If not I might as well just forget that idea and build a totally new PC.
You might notice a small improvement, but not one worthy of the expense. A new motherboard also typically requires a new Windows license as well.
Since it is something you could use in your new build, an upgraded video card might give you the performance you need now, and not be a waste. Something like a R7 260X (<$150) should be a big improvement in games, and should run on your PSU assuming it's not actually a PSU-shaped object such as Sunbeam or Diablotek.
No, you wont be able to keep the same CPU because it can ONLY fit an AM2+. Hence, if you upgrade you will need a new cpu as well. I wouldn't recommend upgrading that.
Instead, you will see a far more noticeable difference in your computers speed if you upgrade to an SSD (not sure if you already have done so). An SSD around 120gb or even 240gb is perfect and prices are lower than ever. It will speed up almost every aspect of your Pc, mainly the boot time and program opening times though, will be much much faster.
So I assume it would work on an AM3+ motherboard however it sounds like the cost wouldn't be worth the upgrade to me.
Another issue with that PC is the lack of ventilation. There is only 1 90mm (I think) exhaust fan and the CPU, video card and HDD can get a bit toasty. That's another reason I was considering the upgrade.
All in all it sounds like it's time to retire this one and start from scratch.