The size of the card is relative. It depends on the information you need to put.
It may have 5.5 cm X 8.5 cm (standard size in Portugal) or may have 4cm x 8cm, 5cm x 8cm, 4,5cm x 9cm, etc...
I usually use a vector program (Illustrator / freehand) and 2 colors enough, but I'm suspicious because I like a clean design with no gradients and other kind of "sh*t"
And as Wiz said: 3mm bleed around the card if you'll use some backgroung. If you use a white background you don´t need it
I usually use PANTONES colors. I don´t know how you make in your countries, but using PANTONE colors (2 only colors) the print will be more cheaper if you'll print on a typography/graphic.
If you'll make it in Photoshop don´t forget use a 300 dpis resolution.
And yes, 300 gsm is the best paper weight for that kind of work
You can check here some great examples:
http://www.behance.net/Search?main-search=business%20card&realm=0&browse=appreciated&category=content
cheers and sorry the english