Still doing research in CF cards as HDD's.  As a note, I thought that was what we were talking about here.  SSDs, technically, should be considered more of a HDD, than a CF card, as they are already designed to work as such (instead of needing an adapter.)  Yes, I know SSD's generally have cache RAM as well, but IDE to CF adapters generally don't have this, though I think they really should.
I've made another interesting discovery.  When picking out a CF card for the adapter, you need to look for ones that are SLC (Single Level Cell,) instead of MLC (Multi,) or TLC (Triple,) which determines how many bits are stored in each memory cell (usually determined by differing voltage levels for M & T.)  With HD style reading/writing, the SLC is faster and much more stable.  With the limits of the ATA-33 interface, most SLC cards max it out with 233x or 300x.  Achieving very close to the same speeds as an HDD on a ATA-33 port.
I've come across another issue as well, which might lead to actually getting a SD to CF adapter and using an SD card instead (haven't started researching that yet.)  CF cards are rated as only accepting 1,000,000 writes per cell before cells start failing.  I don't know how accurate that is, but it really doesn't sound good on a computer you plan to use regularly.  Also, with CF cards, cells are written to faster if they are cleared when they are made available (files are erased/written over.)  The smarter, more modern (ie: expensive,) cards have systems built in to adjust for both of these.  Always re-mapping the cells so that a new cell is written too every time, automatically clearing cells that are re-mapped as free, etc....  This will extend the life of the card (probably by quite a bit,) and allow the card faster writes, but I still don't really like the 1,000,000 write limit.....
edit:  Also forgot to mention that for HDD use, CF cards built on NAND gates are  better than ones built on NOR gates, NAND is better at single bit reads (which is how HDDs work.)
edit2:  Correction, the number is 100,000 not 1,000,000... My bad, sorry about that.  The standard is the same for SD cards (100,000 write cycles before cells start going bad.)  The only advantage I could see in an SD card is that they are generally cheaper (on the searches I did,) and faster than the CF cards.  But the adapter runs around $10-$20 which could mitigate the lower cost of the SD card.