Yeah, if you intend to play Crysis or GTA IV at any decent settings you are going to want to upgrade that CPU.
You can try to bring some new life into your PC with one of the Core2Duos on this list here.http://www.cpu-upgrade.com/mb-ASUS/P5LD2-VM.html
Those are older 65nm versions (they still perform great) and you can find them on places like Amazon.com. They will probably be very expensive "new" since they don't make them anymore, but you might be able to pick up a used one for a decent price...
Pentium D will hold you back quite a bit, a C2D even at 2.4 will cream it, and the Core2 will have good bit of OC headroom if your mobo is up to the task. GL.