Cat 4.12 drivers will not work with the X1600. The card came out long after he 4.12's were available.
Check how many background processes are running Biggles. Also check the CPU usage, while you are at it in the Windows Task Manager, under the "Processes" tab.
A typical Windows XP system should have about 19 to 21 processes running, when you are at the desktop, running no applications. CPU Usage should be bouncing between 0 and 1 percent (best case, occasionally 2%) like a steady heartbeat. You need to watch the CPU usage for a bit. Any spikes are bad news.
Each background process in Windows uses some amount of CPU and consumes other resources. The more processes running, the less CPU power and the less RAM you have to run other things.
Trees will have more impact than virtually any other graphic object in the game. Also, you might consider setting the anistropic filtering to 'application preference' as it really does not do much in the game and will cause the text in the game to be blurry.