On my 98 rigs I do like Phil and use modern SATA with an IDE to SATA adaptor. It allows DMA, and pretty much saturates the motherboards onboard IDE interface. When it comes to 98 the difference between ATA 33 through 133 never really comes into play when using a modern HDD as the onboard cache and vastly superior access time seems to render it moot. The trouble of using a PCI card for a faster interface than what onboard is far more trouble than its worth.
On my pure DOS rigs I use DOM's. Never had a single issue with them. While I used to adore PC-DOS 7/2000, This past year I have switched to MS-DOS 7.1 and find that a partitioned 8GB DOM (two 4GB partitions), is perfect for my builds. You can get them in the common female connector and stick them straight onto the mobo's IDE socket farily cheap, or pay a bit more and get the male version and run two of them on a single IDE cable. I have yet to find a reason to need more than 8GB in a DOS rig, so I just use one directly plugged into IDE 1. This leaves IDE 2 open for an IDE CD/DVD drive and a CF drive (I use the 3 1/2" front bay type as well). You never want to run a CD/DVD drive and HDD on the same channel if you can avoid it.
I would not bother with using a SSD or CF as a 98 drive as the swap file thrashing will kill them way to fast. But you can always use one if you put the swap file on an actual HDD. You will see pretty much maxed out performance and get decent life out of the SSD or CF. I've toyed with CF on 98 in the past and found that while it was super fast, the whole system would pause from time to time due to the slow write speeds and the swap file doing its thing. Reading was fantastic though.
A down and dirty method to speed up a 98 rig is using two good, modern SATA drives. The first one partitioned to 120GB (leave some headroom for the 128GB limit), and install the OS and whatever games, apps, etc you want on it. The second one with a 2 to 4 GB partition then a second partition of 116 to 118 GB for ISO, and other junk. Install one drive on IDE channel 1 and the other on channel 2. Put the swap file into the 2 to 4 GB partition and run as normal. You would be amazed at how fast 98 can be with a setup like that, even if its using an onboard ATA 33 controller.
Just tossing out some options that I've done and found to be very effective.
EDIT
A side bonus of keeping the swap file in its own partition is it REALLY keeps the fragmentation rate down by a massive amount. Defragging becomes something that rarely has to be done with this method.