|My guess is your power supply is failing.|
Video cards and mboards produce little or no smoke while failng - you did say you see no visible damage on those - but power supplies certainly can produce all the symptoms you describe, and it certainly could have damaged the video card and/or the PCI-E X16 slot circuits. If your cpu had produced that much smoke your mboard would not boot at all.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work while failing.
See response 4 in this:
Look INSIDE the power supply.
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minumum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD ! )
If that info is not in the ad for the video card, you can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements. Some power supplies have two or more +12v ratings - in that case, add those ratings to determine the total +12v current capacity.
HD 4830 Graphics System Requirements
450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
"Was playing games ..."
When you're a gamer....
In most if not all cases, the max capacity rating of the PS is an intermittant rating. It's recommended that you do not load your PS to any more that 80% of that rating if you are going to be using something that puts a constant load on it, such as playing a recent game for hours on end. In that case, you multiply the min capacity stated for the system with the particular video chipset on the card by 1.25 to find the min. capacity of the PS you should get.
450 x 1.25 = 562.5 watt min capacity
"Corsair HX 520watt PSU "