or, you could use a 12V relay to switch the AC power to the marquee light.
For example, check out item # 900-2329 on
www.radioshack.com. Less than $2. It has a set of N.O. contacts that can switch up to 10A of 120VAC. Depending on which one you get, the coil can be powered by either 5, 12, or 24VDC (5 or 12VDC can be gotten easily from the PC power supply).
This will draw a lot less current from your PC supply than a chain of taillight bulbs would!
PLEASE DOUBLE CHECK THE CURRENT REQUIREMENTS OF YOUR MARQUEE LIGHT FIRST...THE RELAY MENTIONED ABOVE MAY OR MAY NOT BE SUITABLE FOR EITHER THE CONSTANT OR IN-RUSH CURRENT DEMANDED BY YOUR MARQUEE LIGHT.