"Witch is the difference between "normal" driver and vesa driver ?"
the normal one is the amd or the xorg native driver supporting your x1300, normally supporting 2d and 3d, vesa doesn't support any of that but supports other things present on most cards, perfect for this step
"My motherboard can accept 2 video cards with crossfire system (for ATI, SLI for Nvidia) : for 1 or multiple screens, 1 or 2 gpu could run together in parallel instead of ggu from the motherboard chipset. This run good with 2 identical video cards."
i understand that, what i said is that not much users in linux uses multiple video cards in this type of setup so my knowledge here is limited about the process, your hardware is ok and you can proceed with the steps you mention with the hardware you have
about the hardware, yes, if your mainboard says that is crossfire and both slots are pci x16 or x8 you can do it
what i mentioned was the psu, if it can deliver or not enough watts to make system run stable with the rest of the parts present, remember that you are changing from one old weak video card to two new video cards using probably more power, if your psu is just 350w, you can expect surprises like sudden resets or hard shutdowns without pressing any button
on most sites for two video cards the minimum recommended is a psu at 450 or 500w, from there my recommendation, no relation with the mainboard at all
the 19w you mention is not the consumption of the video card, it is heat generated to the heatsink, look at this link
"Radeon HD 5450 CrossfireX (dual GPUs)
A second card requires you to add another 25 Watts. You need a 350+ Watt power supply unit if you use it in a high-end system. That power supply needs to have (in total accumulated) at least 35 Amps available on the +12 volts rails.
For each card that you add, add another 25 Watts as a safety margin.
There are many good PSUs out there, please do have a look at our many PSU reviews as we have loads of recommended PSUs for you to check out in there. What would happen if your PSU can't cope with the load?
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down"http://www.guru3d.com/articles_pages/radeon_hd_5450_review,6.html
they mention a 350w real psu, i have some here that says 600w but in reality those are 300w only, if you have a good thermaltake or similar good brand psu with enough watts then you should be ok, considering that i don't know the rest of the hardware you have like cpu, hard disks and similar stuff using power from the cpu
you didn't mentioned why you choose two weak video cards over one stronger