According to linuxwireless.org the in-kernel Ralink drivers do support AP mode on the older chipsets, both PCI and USB.
If you're still using Debian Lenny, you will have an older version of the drivers which might be affected by bugs or other limitations. As Lenny is currently the oldstable, I would recommend upgrading to the current stable distribution aka Squeeze, if that's possible.
Otherwise, compat-wireless provides a chance to install the most recent drivers on a system without disturbing the rest of the system (this holds true for Squeeze as well)
Debian does not provide ready-made packages for compat-wireless so you will have to download it from linuxwireless.org and build it yourself. That's very easy, the linked page explains the simple few steps needed to do it. Also, the operation can be reverted.
As far the firmware goes, there have been reports of the package in Debian being quite outdated, that is true for squeeze too. Luckily, you can just get the most recent firwmares directly from Ralink.