A T-1 is a dedicated phone/data connection supporting data rates of 1.544Mbits per second. A T-1 line actually consists of 24 individual channels, each of which supports 64Kbits per second. Each 64Kbit/second channel can be configured to carry voice or data traffic. Most telephone companies allow you to buy just some of these individual channels, known as fractional T-1 access.
If you currently have a T-1 you would want to find a way to monitor current bandwidth during your most active usage times. Most newer routers or firewalls have viewable "real-time" charts or graphs that will show you bandwidth usage via a PDM (Cisco) or web interface. You maybe surprised to see that you don't use much bandwidth at all. If you experience lag while using streaming audio or video, VoIP, email services such as MS Exchange, or online file backup services you may want to concider speaking with your ISP and implementing QoS (Quality of Service) between your border gateway router and your ISP's router on the other side of the T-1. Quality of service is a way for the hardware to determine what type of traffic is consuming bandwidth and prioritizing it so that it would remain uninterupted by other traffic to help avoid lag or bandwidth spikes that are often experienced during file transfers. Unless you have and extreme amount of users, host high volume webservers or run bandwidth intensive SOA web applications you should have adequate bandwidth running a single T-1.
If you've determined that you're maxing out your bandwidth and are interested in utilizing Multiple T-1's, your hardware need will ultimately be determined by how you would like to use the T-1's. You can configure end hardware to perform load balancing, load sharing, connection fail-over or combinations of balancing, sharing, and failover techniques. When speaking with T-1 service providers about multiple T-1's the terms used are either going to be "NxT1" or "Bonded T-1". You will require a CSU/DSU (aka "Port") and router (unless using a specialized bridge) per T-1 at each end of the connection so be prepared for additional equipment costs or port fees. (A lot of the times these port fees are included in the overall cost.) The equipment brand is irrelevant, just be sure that the administrator will be able to use it and make necessary configuration changes. Also, be sure that your hardware can support QoS for traffic prioritization (You'll thank me in the long run). Another connection option to look in to would be a MPLS VPN. Depending on your service location and how far it is from the nearest POP (Point of Presence) this MPLS option could be a more inexpensive option than a NxT1.
In situations when you are looking to expanding your bandwidth capacity it may be worth while to hire a consultant to come to your location and measure your peak usage to determine if the addition of additional lines is necessary.
Good Luck.