Reaktor/Hardware/Soundcards

General

Regardless of the sound card you choose, if you use Windows you will have to tweak your system to get smooth low latency operation. Some basic tips:

If it's a PCI sound card, make sure it's not sharing an IRQ with anything. In the device manager, choose view resources by type and make sure nothing else is on the same IRQ as your card. If there is, you might try moving it to a different PCI slot.
Turn off screen savers, virus scanners, firewalls, Palm Pilot sync and any other memory resident programs that might interfere. A hardware gateway/router that does NAT is a good idea if you have a highspeed connection - running a Windows-based computer on a highspeed connection with no firewall at all is suicidal.
DO NOT lower the hardware acceleration on your video card as some resources have suggested. Running in 16 bit color could help. more info
Make sure you have the latest drivers for your unit.

Echo Indigo DJ Portable CardBus card for a laptop, doesn't require power supply, very low latency ~5ms, two independent outputs (line, phones), non-exclusive driver, which allows to run multiple sound application at the same time and mix their signal on the card
RME Multiface 8-in 8 out interface with PCI card or Cardbus, very low latency ~5ms
M-Audio 24/96 An inexpensive low latency solution, good for beginners. ~5ms or less with a bit of tweaking.
01Xray site dedicated to the 01XThe yamaha 01X is ideal for Reaktor as not only do you get a low latency soundcard ~5ms or less but yo also get a control surface which is great for tweaking all those knobs and faders in Reaktor. RRP £1299 although it is available for about £1000 from most retailers.