Wich RAM card should I get?

You could probably go 1 module and save up for an upgrade later but there is a catch. Some motherboard have a feature called Dual-Channel RAM, meaning if you have modules of the same type, size, and latency you can get faster speeds out of it.

If anything go for pairs of RAM modules. Newegg.com always has some for cheap at times. Kingston ValueRAM is inexpensive but has great performance I use it a lot for my customers.