Psu and graphic card

Starting from top to bottom:
Yes, your PSU is able to run the card, depending on what other hardware you have also makes a deference in the power required to be supplied by the PSU

The 7950GT 512MB needs a minimum of a 350w PSU, but NVIDIA recommends a 400w or higher for optimal performance of the card

On the problems side:

The green and purple stripes can be created by a number of problems, but the most common is faulty hardware

The screen being pixilated can be 3 things, a wrong driver or no driver at all, screen resolution to low for the screen you are using, or yet again, faulty hardware whereas the first two is more likely

The system not being able to boot into windows could be related to the driver not being installed and/or the card not getting enough power, it can also be related yet again to a faulty card, but it being a Driver is this case most likely in this case, and since you can boot into save mode, see if you can see any driver files left over by your X300, it isn’t uncommon for files to say behind after the program or component has been uninstalled

The devise being stuck in an infinite loop can and most likely be caused by the drivers as well, since windows has a default *no extras* driver built into the system, is can cause the problem because the instructions that the card uses are not included in this deavult driver

Save mode does not stably support any higher resolution than 640 x 480 (up to 800 x 600 with SP2 installed) and can/will cause problems if set to any higher

Weird color pattern can be cause by a few things as well, the problem could be with your screen not displaying the color properly, it could be a faulty driver giving wrong color information to the card, or it could be the card itself, with the driver not being installed seems the be the culprit in this case

Recommendations:

Test the card in another computer or multiple other computers (if able) and see if the problem arises with the other one as well, that way it can be determined if the card is working properly, try a bigger PSU, the card might not be getting enough power the root of all the above problems, if possible, try and download the latest drivers for your card, preferably the WHQL certified forceware driver and try and install id under Save mode(it is possible) while in save mode, see if any files are left from the previous drivers and remove them, reboot and see if problem persists, A windows repair might also be needed in some cases, if there are up-to-date backups of your pc it is a good way to sort out certain windows booting problems, or take the card back to the place it was bought and let it be tested under certain controlled conditions if it comes to that a format and clean install of windows might be needed, but that is a last-resort fix and should be avoided if possible

This is great advice, but to further it, your card may require external power support, in addition to power from PCIX bus. So look for a power adapter on the card, and see if your PSU has a power cable for it. If not, you really need a more modern PSU. If you do, I strongly recommend Antec brand ones. Do not get a cheap PSU for $10 or $20, they can explode (literally). Its an overgeneralization, but basically you want a nice PSU for your expensive vid card and MB.