If that's the case, the answer depends on if you plan on doing any serious overclocking or not.
When it comes to DDR ram, you really only have 3 real choices as far as timing options go. There's also the option of getting ram capable of running faster than PC3200 (400 MHz speeds).
Remember that a Pentium 4 can only be overclocked by changing its FSB (front side bus) clock. Your 2.8C has a 200 MHz QDR (quad data rate) front side bus running at 200 MHz. (QDR makes it 800 MHz effective rate as far as bandwidth goes.) That means the CPU has a multiplier of 14. (200 MHz x 14 = 2800 MHz) What this means is that if you increase the CPU FSB clock by 1 MHz, you will overclock the CPU by 14 MHz. It also means your ram will have to run 1 MHz faster. This means that you would also be overclocking your memory with a 1 to 1 FSB to memory clock divider. (Which is the default when using PC3200 memory.) This is why some people buy PC3500 or faster memory - so they can leave the divider at 1:1 and increase the CPU FSB and still have the ram running within specs. The second option used is to go into the bios and select a different divider, like 5:4. If this divider is used then when the CPU is at 200 MHz FSB, the ram will be running at 160 MHz. With this divider, overclocking the CPU FSB to 250 MHz will make the ram run at the correct 200 MHz used for PC3200. With the FSB set to 250 MHz, a 2.8 C P4 will try to run at 3.5 GHz, which is highly unlikely without extreme cooling. The other thing to consider is that a system will perform best if the ram is syncronous with the processor's FSB (or a bit faster in some cases).
Now back to the question at hand, what ram should you buy? If you don't plan on overclocking the CPU, you should buy PC3200 (DDR400). If you plan on doing any serious overclocking, you should buy something faster than PC3200 unless you want to overclock your ram by setting a lower divider in the bios. If you do this (set the lower divider), you won't gain nearly as much from the overclock.
The other option on ram is CAS latency. DDR400 memory is available with CAS latency ratings of 2, 2.5, and 3, with 2.5 being the most common. RAM with a CAS latency of 2 is 1 cycle faster than that with a CAS latency of 3. (For DDR400, this would be 1/(200 Million) seconds saved for every memory access.) Is this a big difference? No, it is an extemely small performance difference; however it is often true that memory with a lower CAS rating is also of higher quality than ram with a higher cas latency, so there's more to it than meets the eye. Usually ram with a lower CAS latency will overclock better than another stick of ram with a higher CAS latency.
Unfortunately, this might possibly be confusing to you, but that's as simple as it gets from a technical perspective. Basicially, if you don't plan on overclocking, just get regular PC3200 (DDR400) CAS 2.5 or 2.0. If you plan on overclocking a little, the CAS 2.0 memory would probably have a bit of overclocking headroom more than CAS 2.5 memory to run a little over 200 MHz. For really serious overclocking, you would want to get PC3500 (DDR433) or faster. Consider that for best performance you will need 2 sticks of memory to take advantage of Dual Channel operation on a 'C' type P4. This means for 512 MB of total ram, you will want to buy 2 256 MB sticks of memory. For 1 GB of total ram, you will want 2 512 MB sticks.
