I am having this very same problem after upgrading to Windows 7 x64. Latest firmware on the DIR-655. I have Auto Negotiate already selected, connecting at 1.0 Gbps. Have tried many suggestions on the 'net. DLink needs to take a very serious look into compatibility between their DIR-655 and Windows 7...
Abit AB9 QuadGT Motherboard
Latest NVIDIA Drivers
Latest Firmware on DIR-655 (older model, not the newer hardware)
I used to be able to transfer files at MegaBytes per second on this machine. Now it is KiloBytes per second. I've been able to boost it from 30KB per second up to 200KB per second....still sucks. Will start playing with Flow Control and Negotiation next, however if I can't connect at 1.0 Gbps reliably I am going to be extremely pissed.
Edit: Updating Realtek PCI GBE driver via Windows Update (Drivers Tab, Update Driver) has yielded almost 600 KB/s. Progress, but I expect far better than this.
Edit: This problem is affecting file transfers between computers, Remote Desktop, Download Speeds...the whole 9 yards.