It’s been ages since I’ve been doing stuff with hardware, but the computer beeping on startup can mean different problems ( depending on the number of beeps etc )
Things such as the the graphics card can’t be found ( which is why it beeps, as it may not be able to show the error ), or a problem with the memory.
It’s weird that you have a voltate low error message relating to your system battery ( which is used to store BIOS information, such as date, info on the hard drive etc ). Also, it not finding your graphics card, especially after re-seating it, is strange.
It might be worth replacing your power supply unit… if it’s faulty, it may be the reason that the voltage error appeared, and if it is gradually getting worse, it may not be able to power the ( probably souped up ) graphics card.
Hope that helps a bit! If it does, you owe me a few of those pints from the wedding up in Derry back :P