Day trading apps are usually Java memory hogs that support multiple monitors, some running streaming audio/video. I was thinking of just using the enthusiast build from the latest SBM...
Processor: Intel Core i7-930
Motherboard: ASRock X58 Extreme
Cooler: Rosewill Fort 120
Video Cards: 2 x Radeon HD 5850 in CrossFire
Power Supply: Corsair CMPSU-750TX
Case: Cooler Master CM 690
Memory: Crucial 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3-1333 Triple-Channel Kit
Hard Drive: Western Digital Caviar Black 750GB
Optical Drive: OEM Samsung SH-S223C CD/DVD Burner SATA
APPROXIMATE PURCHASE DATE: Today
BUDGET RANGE: ~$1500
SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Trading apps, MySQL, Office
SLI OR CROSSFIRE: Yes
MONITOR RESOLUTION: 1920x1080, 1920x1200
ADDITIONAL COMMENTS: Needs to support 2-3-4 monitors.
Mobo: A higher quality board. It also supports USB 3/SATA III, so it's more future proof and will be faster to back up.
RAM: Higher quality, faster RAM. Also better for overclocking.
GPU: You don't need 5850s. The 5770 would be much better. 5850s are only really good for gaming at 1900x resolutions, but you're not doing anything that's graphics intensive.
SSD: You put your OS and programs on this drive and it will speed everything up. This is almost a necessity for day trading, as split seconds matter. As this won't store any data, it might be too big for your uses. You could easily drop down to an 80 GB one and save a good chunk of money. Keep in mind that SSDs like to have at least 20% free space, which needs to be accounted for when determine the size you need.
HDD: Again, a much faster drive. If you need more space than this, they make a 1 TB model for $90.
Case: A high quality case that's cheaper with the PSU, which is also very high quality.
HSF: A much better cooler.