Since you have 3 PC's uploading to the Internet constantly, the bigger question is, what DSL router and switch are you using? Are you sure it's a switch and not a hub? Hubs divide the 10/100Mbps connection to each port, depending on which ports demand more at a given time; whereas a switch dedicates a full 10/100Mbps to each port at all times. You definitely want good networking equipment, and I'm not sure how good Gigafast is; I've never heard of them, actually. Why not get a 4, 7, or 8-port DSL Router/Switch? SMC, D-Link, Linksys, and Netgear make such routers/switches for a reasonable price. I'm very happy with my Netgear RT314 even though it is considered old technology now.
If you are transferring files from PC to PC on your LAN and you notice bogging down, then definitely get a 100Mbps switch. You will notice a big difference between 10 and 100Mbps LAN connections. However, it sounds like you are suffering from poor bandwidth because your DSL connection is the bottleneck. 256Kbps is not much compared to 10Mbps. Unless you get a 10 or 100Mbps connection to your ISP, you will not be able to utilize your switch's full bandwidth capabilities to the Internet.
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