An Athlon Thunderbird 900 oced to 1066mhz and your wondering why your FPS is so low in UT2k3? To answer your question, no you should not expect 50 - 60fps with those specs. Your computer is too slow. Furthermore I'm assuming this is an FX5600 non-ultra ? That's the equivalent of a GF3TI in performance so don't expect the world.
Sorry man, but your asking too much of your system. Get yourself a Barton if your motherboard can take it, or at least get the maximum AthlonXP it can handle.
Athlon64 3200 Clawhammer / Asus K8N-E Deluxe / 1GB Crucial DDR400 / Geforce 6600GT 128MB AGP / Windows XP Pro