Well, the CPU support list for that motherboard lists the Phenom II X4 955. It's about $100 and would be a considerable upgrade from the 545. There's some others, too. Take your pick. Keep in mind you might need to upgrade the BIOS first before swapping CPUs.
M4N98TD EVO CPU support list:
https://www.asus.com/Motherboards/M4N98TD_EVO/HelpDesk_CPU/