We don't have any percentages or numbers for how much a condom decreases your risk of catching an STD. However, we can say that condoms do decrease your risk for catching and transmitting STDs. Additionally, your risk factors would depend on the activities you were engaging in, and how properly you used the condom.
Here are some articles that have more info about stds and safer sex practices:
A Quickie STD/STI Risk Assessment
Safe, Sound & Sexy – A Safer Sex How-To
Only you can decide whether it's purely numbers that will make a significant difference in taking responsibility for your sexual and overall health.