I don't have any suggestions for colo in Dallas, but you'll probably end up paying more for colo than you would just to rent a dedicated server from somewhere like Server Matrix. They have systems starting around $100, colo typically starts in the $200-300 range and you're typically charged per megabit based on sustained bandwidth. If I did my calculating correctly, 500gb a month averages to about 200kb/s 24/7.
If you're set on colo, I've had the best luck making friends with a local ISP or a systems admin at a local company with a good connection. Usually you don't have much luck with the companies letting you colo, but most small ISPs will welcome whatever extra money they can make. Although these small ISPs are likely to kick you to the curb when they see how much bandwidth you're sucking up. 8O