Yes, it's 256MB.
Yes, it's 256MB.
Comparing a single 512MB DDR3 chip versus two 256MB DDR2 modules is a common question. The performance depends on the specific system and workload, but generally, a single high-capacity DDR3 stick can offer better speed and stability compared to splitting memory into smaller sizes. For your 98SE build, going with one 512MB DDR3 should be more efficient than using two 256MB DDR2 sticks.
The quickest DDR2 available offers speeds around 8533 MB/s, while the slowest DDR3 reaches about 6400 MB/s and can go up to 12800 MB/s. In general, DDR3 tends to be quicker than DDR2. Using dual channels can boost your theoretical bandwidth by roughly 10–15%.
Dual channel could offer better performance, especially with a motherboard supporting it. DDR2 aligns more closely with hardware from around Windows 98. By the time DDR3 emerged, Windows XP was already available. I recall using an AMD XP machine when XP launched, and those systems ran on DDR2. Prior to that, boards featured three DDR1 slots, so I stuck with two 128MB units—adding a third would disrupt dual channel and slow things down. It seems my setup was likely a Super Socket 7 system (AMD K6 2 3D-NOW).
We're sure you can get 98SE running on DDR3, right? We thought PCIE and 98SE were out of the question, but now we see otherwise. It might even be possible to use SLI. The latest AGP cards I found are AM2 or 939. You could run a Core 2 Quad 775 with DDR2 using PCIe—it's pretty straightforward since only one core is needed. The best single-core CPU with the most money would likely be the fastest option. Overclocking could find an M.2 slot that supports it, in theory. Heat management isn't too much of a concern these days, especially with good cooling solutions.
This advice suggests that high-end DDR2 might be limited or costly, similar to what you'd expect for 512MB DDR3, so sticking with DDR2 seems reasonable. You could also consider a quad-core processor using DDR2 with PCIe SATA. The RAM should match the same frequency, and timing will matter as expected. Edited June 29, 2021 by thrasher_565
98 can run smoothly only with drivers present, otherwise it won't function properly. 98se becomes unreliable after using more than 512MB of memory.
The errors are called "it won't boot". There's a reason that people aren't using modern DDR3 platforms on 98, and it's because when you go that new there are no chipset drivers available for 98SE. The latest that you have drivers for are early LGA775 boards with DDR2. RAM speeds at that level will not influence how Windows 98 runs, as you're already running it on very overpowered hardware. Quad cores won't help either, as 98 cannot use multiple cores. PCI-e is not very stable on 98, and you're not gonna find a board that has 2 x16 lanes, and to top it off people have already tried nVidia SLI in 98. Not gonna happen.