Check in the hardware troubleshooting section where many such configs have been given... Also, you have not informed what are all to be included in the budget - monitor, UPS, any printer, speakers?
Anyway here is my recommendation
Motherboard: Gigabyte GA-MA78GM-S2H: 4250
Processor: 	AMD X2 - 5000+          : 2850
Memory:  2x1GB DDR2 800MHz              : 1600
Hard disk: Seagate 320 GB SATA HDD    : 2250
Optical drive: Sony DVD-Writer 20xD      : 1150
Cabinet and SMPS: As per your choice   : 2000 (say)
Monitor: 17" LCD                                : 7300
Logitech Keyboard and Optical Mouse     : 700
Sub Total: 22100
There is no need to go for a gfx card if you are only a casual gamer since you have the 780G based motherboard... If you need an UPS, that will set you off by about 2k for 500-600VA models. If you need a speaker set, you can get a basic set at 500 to 1000, or you can go for a 5.1 setup at around 3.5k. If you go for CRT monitor instead of LCD monitor, 17" should come at about 2 to 2.5k less...
After factoring all of this, you can figure out how much extra you have and put it into getting a higher proccy... 
Arun