CPu support list
KC.AE202.240 CPU.ATH2.240e/2.8G/2x1M/45
KC.AE202.405 CPU.ATHII.405e/2.3G/3x512K/45W
KC.AE202.605 CPU.ATH.605e/2.3G/4x512K/45W
KC.AM202.170 CPU.ATH2.170u/2.0G/1M/45
KC.AM202.220 CPU.ATHII.220/2.8G/2x512/65W
KC.AM202.435 CPU.ATHII435/2.9G/3x512K/95W
KC.AM202.440 CPU.ATHII440/3.0G/3x512K/95W
KC.AM202.445 CPU.ATHII445/3.1G/3x512K/95W
KC.AM202.635 CPU.ATH.635/2.9G/4x512K/95W/C2
KC.AM202.640 CPU.ATH.640/3.0G/4x512K/95W/C3
KC.AM202.645 CPU.ATH.645/3.1G/4x512K/95W/C3
KC.AT202.215 CPU.ATH2.215/2.7G/2X512K/65W
KC.AT202.240 CPU.ATHII.240/2.8G/2x1M/65W
KC.AT202.245 CPU.ATHII.240/2.9G/2x1M/65W
KC.AT202.250 CPU.ATHII.250/3.0G/2x1M/65W
KC.AT202.425 CPU.ATHII425/2.7G/3x512K/95W
KC.AT202.435 CPU.ATHII435/2.9G/3x512K/95W
KC.AT202.440 CPU.ATHII440/3.0G/3x512K/95W
KC.AT202.635 CPU.ATH.II.635/2.9G/4x512K/95W/C2
KC.AU202.160 CPU.ATH2.160u/1.8G/1M/45
KC.AU202.260 CPU.ATH2.260u/1.8G/2x1M/45
KC.PE202.900 CPU.PHEN II.900e/2.4G/2M/65W/C2
KC.PH202.35T CPU.PHEN II.1035T/2.6G/6x512/95W/C3
KC.PH202.45T CPU.PHEN II.1045T/2.7G/6x512/95W/C3
KC.PH202.545 CPU.PHEN2.545/3.0G/1M/80W/C2
KC.PH202.810 CPU.PHEN II.810/2.6G/2M/95W/C2
KC.PH202.820 CPU.PHEN II.820/2.8G/2M/95W/C2
KC.PM202.545 CPU.PHEN2.545/3.0G/1M/80W/C2
KC.PM202.830 CPU.PHEN II.830/2.9G/4M/95W/C3
KC.PU202.55T CPU.PHEN II.1055T/2.8G/6x512/95W/C3
KC.PU202.65T CPU.PHEN II.1065T/2.9G/6x512/95W/E0
KC.PU202.945 CPU.PHEN II.945/3.0G/95W/C2
see more here...
http://community.acer.com/t5/Aspire-Desktops/Acer-Aspire-x3400-cpu-upgrade/td-p/351129
That said I would go with the Phenom II 1065T