Older SATA cards may work. They often are SATA 1 cards with IDE controllers onboard. Look for something with booting support, not just for your BIOS, but also with a driver for Windows 98.
IDE drives were NEVER the fast ones, and it's somewhat still the case today. SCSI has always ruled the roost in high end performance drives. IDE is limited to 133 mbps at the fastest, with a drive that can even support that, while an Ultra320 SCSI controller from around the same time can go up to 320mbps. There are also things like 10 and 15k RPM drives that will improve speed dramatically. SCSI does happen to be considerably expensive, even today, but it's mostly in terms of controllers. If you can find an Ultra 320 PCI controller that supports booting and has a Windows 98 pre-install driver, you may be able to find SCA-80 drives for next to nothing. They are fast, reliable, and will probably last 50 years from new. While this is all well and good, even the cables are expensive as it's all server gear that's not being ripped out of anything anymore.
IDE to SATA adapters are things I have not tried, but it really depends on the throughput of the adapter. It will still be limited to that 133mbps (around 16 megabytes per second) speed, even with a good drive.
SSD Trim is an automatic function in most cases, afaik. It's supported by various operating systems, and not by others, but if you're talking a modern SATA SSD, the speed decrease you would get from no trim function would still be faster than anything on even a 15k Ultra 320 drive.
There are also SD and CF adapters, but I personally dislike them. It just feels weird to have a format that's intended to be completely mobile rooted to a machine. It just takes some of the oldschool magic out of the machine.
My suggestion is to get an IDE drive. They are slow, but that's part of the fine computing experience. The anticipation of waiting for games to load just tells you what that period was like. SATA is more for stuff like XP, but even to that matter, most Socket A chips are really a bit past Windows 98, and more in the Windows ME territory, which would probably have better SATA support. Even Slot A would do well with 1999 ME. Don't be scared by the whole "Windows Crash Edition" thing with ME. It's a usable OS.
If you are looking for the fastest thing on planet fast, get a board with PCI-X, 64-bit PCI, or 66+mhz PCI. These are faster versions of PCI, normally available on workstation and server gear, but drive controllers can normally take advantage of that. You can also try a RAID array, with something like a 4 drive RAID 1/0 combo. That will definitely be faster. SATA is going to probably have driver issues, because it came out in 2003, well after XP hit the scene. SCSI is fast, but expensive, and the IDE/SATA converters may be a mixed bag.