If you still have your old midtower casing, then I suggest keeping it. As for the monitor, buy something with a resolution of 1680X1050 or below to prevent any hiccups. For visual eye candy, and since this is a new system, 22" and with at least a resolution of 1600 x 900 or 1680 x 1050 is a must hehe.

Prices are based on my copy of PCExpress and PCHub's latest.

Intel Build:

CPU: Intel E6300 (C2D w/ 2MB L2 Cache) 2.8Ghz: 4,100
Board: Asus P5QL-SE P43 or any P43 board: 3,700
GPU: Palit GTS 250 Green Edition or Sapphire/Powercolor Radeon 4770: 5,770
Memory: DDR2 2GB 800: 1,550
Hard Disk: Seagate 500gb 7200.12: 2,550
Power Supply: Acbel Ipower 610w: 3,300
Monitor: Dell 22" e228wfp or any 22": 9,000

Total: 29,970

AMD Build:

CPU: AMD Athlon II X2 (Phenom II X4 with 2 cores and w/o L3 cache) 2.8Ghz: 3,150
Board: Any AM2/AM3 board with an SB710-SB750 southbridge : 3,200 - 3,800
GPU: Sapphire/Powercolor Radeon 4770: 5,550 - 5,770
Memory: DDR2 2GB 800 or DDR3 2GB: 1,550 - 2,340
Hard Disk: Seagate 500gb 7200.12: 2,550
Power Supply: Acbel Ipower 610w: 3,300
Monitor: Dell 22" e228wfp or any 22" : 9,000

Total: 28,300 - 29,910

Try playing around with lower monitor and motherboard prices and see if you can increase your budget a little bit. A Radeon 4850 512mb can be had for as low as 7,000.