you can try ikea... for cheap modern designs. there's one in the east, and one in the west...
http://www.ikea.com.sg/about_ikea/map.asp
for more expensive furniture in general, you can also try Park Mall (near Dhoby Gaut MRT Station).
Else, IMM and Furniture Mall has a quite a fair mix of shops, but some people have bad experience with some shops. I guess you should always go around to have a sense of the prices before committing... and never pay in full... a 10% deposit should suffice in most situations, the rest you can pay upon delivery.
hope this helps.