What's the maximum speed of an Ethernet cable (not a phone cable! the one that has 8 threads)
Gigabit Ethernet (1000BASE-T) utilising a standard cat5 ethernet cable is cabable of 1,000 Mbit/s (mega bits per second) or 116 MB/s (mega bytes per second), although achieving this is practice can be troublesome, it's almost possible to achieve using two machines with a crossover cable between two high quality (i.e. Intel) network cards. You can always use multiple network ports/cards in an aggregated or bonded configuration to achieve multiples of this bandwidth (the differences between them are slight, the results differ based on usage - redundancy, throughput, error handling etc).
About the Gigabit Ethernet Network Card: Is it just a technology name or can reach 1 GigaBit / Second
It does what it says on the tin - 1 Gbit = 1,000,000,000 bits, and this is the throughput per second. Notice the difference in Gbits and Gbytes - a factor of 8 (1 byte = 8 bits).
Does computers really reach the maximal network card speed?
This depends on the quality of the network hardware (including the Network Interface Card), intermediate routers, cabling quality, noise/interference from other electrical components (this is unlikely except in the case of very "noisy" components and unshielded cat5) and network load (if you are sharing routers/switches with other devices). You may also want to enable technologies such as Jumbo Frames to increase throughput.
Most Important Question (I want to answer):
Let's assume I have a computer that come with a 10 Mbit/sec network card (an old computer of the era of those network cards), is replacing this network card with a Gigbit card a bad or good practice? Will it speed up the data? or will it plant the computer because it's a super card?
The gigabit card will go faster IF you connect it to gigabit capable equipment (a router/switch or another machine with a gigabit card). As noted by nik you require enough bandwidth on the interface which the card is plugged into (PCI, PCIx etc). The PCI bus has 133.33 MB/s bandwidth and will support the speeds required for gigabit ethernet.
Here is an article Gigabit Ethernet: Is the Time Right? for a business specific implementation.