You need to understand a few things about your system.
The P4 is a highly inefficient CPU. One way to partially make up for that inefficiency is to run the RAM at a higher frequency than the CPU. For CPU's other than the P4, the optimal CPU:DRAM frequency ratio is 1:1, but for the P4 it should be either 3:4 or 4:5, depending on the base frequency of the CPU you're working with. In the case of a 533MHz FSB P4 (133MHz freq), you should be using the 4:5 ratio. That would put the RAM freq at 166MHz (DDR333, aka PC2700).
If you were to get an 800MHz FSB CPU (200MHz freq) to pair up with your PC3200 RAM (200MHz freq), the ratio would be 1:1. Your system would perform better due to the FSB increase from 533 to 800MHz, but it wouldn't perform optimally because the ratio wouldn't be optimal.
You have to look at the BIG picture. You have a board that's based on a VIA chipset, a 533MHz FSB CPU & PC3200. So not only is your system obsolete, it's poorly configured. The saving grace is that both the 2.8GHz Northwood & your board are highly overclockable. So rather than wasting your money on a new CPU, why not just overclock what you have & optimize the settings at the same time?
But before you proceed, you need to get the temperatures under control. The board of 41C is high...what is your average room temp? And 85C for the CPU is extremely high...you're at the point where the CPU will begin throttling. Where are you getting these readings from...software? Check the BIOS. I suspect those readings are incorrect.