Just an update as far as the SIL3512 card.
It worked wonders when I plugged them in. The BIOS recognized it just fine and I had the Windows 98 SE drivers on hand and detected it and installed just fine. I ordered two 120GB hard drives to see what I get. At that time though, I actually didn't have any SATA power connectors, but was able to order a molex to SATA power converter from EBay for really cheap.
So anyways, I plugged the drives in and detected them just fine. Was a bit finicky though at first. When I first got the card, the bracket was way too short for to fit in my system. So, I just swapped it with another bracket from another PCI card and works just fine. Plus, I did have a bit of a hard time trying to get the drives to work properly, but it ended up being me setting up the drives physically in a weird way.
But anyways, I've installed the drives into the system as normally and was able to partition both of them with the Seagate Drive Overlay Software (Windows 98 SE FDISK was completely useless. Wouldn't partition the entire disk.). I got Disk 1 with about a 30GB partition and around an 80GB partition (apparently both drives were detected as 111GB, but no matter), and I've got Disk 2 partitioned fully. Booted it into the DOS prompt and worked just fine. Installed Windows 98 SE on Disk 1 and so far no hiccups.
The only thing that I need to find out is if the DMA capabilities are actually present on that card, as Phil pointed out in one of this YouTube videos. Another thing too is that while the SATA card is bootable, I can no longer boot from my IDE hard drives first. But since I've got my first disk partitioned into two logical drives, it shouldn't be a major issue. I'll update this further as I reinstall everything on my system.