I have a system with a 300 watts PS and I've got an old ATI 5750 from a friend. Can I just use a second power supply (I read about the paperclip trick) to power the video card and leave the PS outside the case? I am not an expert and too lazy to mess all the wirings inside. Thanks.
Well, the possible and the recommended are two totally different things. I recommend you upgrade your PSU. If you are going to go ahead and ignore the recommendation then ensure that the two PSU's share the same electrical ground and install a power switch on the external one, perhaps only loading the external with devices other than the Mobo/GPU.
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard 450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ playback requires Blu-ray drive
For an ATI CrossFireX™ system, a second ATI Radeon™ HD 5750 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required
The recommended PSU is for the entire system. IMO is better go with one PSU from a good brand that meets the power requirements.