I'm using 2x Delta 1010 and 1x Delta 66 in Windows 7 Home Premium x64 with no problems.
If you're looking for more inputs, this is a very plausible upgrade option for you, and if you buy from a vender with a generous return policy, it's worth a shot.
Some motherboards are more forgiving than others about running Multiple cards, as it's a hardware issue regarding IRQs and resource sharing, but if it works well for you, I wouldn't think into it at all.
If you wanted to try purchasing a used card from ebay or any other place, it could give you an idea of how well it will work without spending price of a brand new card, in the event you would have problems.
Core 2 Quad Q9400-Scythe Ninja Rev A|MSI Neo P45|WD 150GB Raptor|WD 250gb SATA-I|8GB G-Skill DDR2|WD Blue 640GB SATA-II|Logitech KB|Apple Mighty Mouse|ATI HD4850 512MB-Accelero S1|M-Audio 2x Delta 1010|Delta 66 Omni I/O|Corsair 400W|Cooler Master Elite 330
Dual Boot: Ubuntu 10.04 x64 Lucid/Windows 7 Home Premium x64
Adobe Audition Mac & PC, Logic Pro, Ardour, PT 9, Reaper x86 & x64, Ableton Live Lite
2x Delta 1010, 2x Delta 66, AP2496, FW Solo, Pulsar II, ProFire 2626, Fast Track C400