I'de like to upgrade my computer for gaming purposes. It's just that i'm not sure what i should do/buy in order to do so.
OS: Windows 32 bit
Computer specs:
CPU: AMD Athlon II X4 645
Mainboard: FOXCONN 2AB1
Memory: DDR3 6GB
Graphics: Radeon HD 4200
 
Really NEED help, would like to play like "Fallout 3", "Fallout:New Vegas".  I can play fallout new vegas on lowest setting with slight lag. Would like to atleast play with no lag. My budget is a maximum of 100 dollars!

Buy a better video card, everything else looks like plenty. I have a slightly slower CPU the Athlon II x4 620 2600Mhz and it plays games well with a good video card. The video card you have is weak!

How many watts and what is the make/model of your power supply? You may need a larger power supply to handle a better video card.My psu is 250 watts.Power Supply will need to be upgraded to at least 500 watts. This cuts deep into the $100 budget.

Here is a suggestion:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817438016 EVGA 500Watt PSU $37 after rebate.

http://www.newegg.com/Product/Product.aspx?Item=N82E16814130791 EVGA GeForce GT 620 $49 after rebate.

One of my friends has the GT 620 and it works well with games like WoW, Skyrim, GTA, etc... so it should be ok on "Normal" graphics settings to play the games you want, HOWEVER not sure if it will play well on max settings, however its FAR better than the ATI Radeon 4200, so if the game is running in the Radeon 4200 now, it will be way better with this card! However this system would probably not boot and black screen on you if you try to get by with that 250 watt power supply! I have seen some people get lucky on a 350watt PSU and a video card SIMILAR to this one installed, but its running the powersupply to the max if it works at all and you are prone to crashes when the power supply CANT keep up. So 500 watts or better is best.

I have one system that is running on a 460 watt power supply and a ATI Radeon HD5450 and I have to upgrade my power supply to install a better video card. Probably GOING to go with Corsair 650 watt because I have a higher end over clocked and power hungry video card that is going to be an upgrade for my system. nVidia claims that the video card I have requires a minimum of 500 watts, but its good to have more than less especially when you have extra fans and drives installed to draw more power from the system than a regular workstation build.
 Thank you so much!  My graphics card atm wouldnt even play minecraft(Didn't support open GL 2.0 ) :_( Thank you so much! XDPersonally though I would wait and get a better GPU than a 620 - It'll handle basic games okay but is really nothing special and is certainly not a "gaming" card.  If your budget is really tight I'd consider seeing what cards you can get 2nd hand as you can probably get something that fits your budget that is already miles better than the 620.