Be very cautious when you buy form any auction site, such as eBay as buying any electrical equipment is a bit of a gamble. You have to really cover the groundwork, checking out the buyer etc and should never rush no matter how much of a good deal you perceive is on offer.
Having said that, I'm sure there are many others who have been more than happy with their 2nd hand macs that they won on eBay and I am one of those ... but it is definitely Buyer Beware!
I
*think* you are looking at the "Sawtooth" Power Mac G4 AGP Graphics model which was released back in Sept '99.
I have no clue as to the difficulty of upgrading a processor ... and as for converting to Dual, I'm afraid I can't answer that as I honestly am not sure.
But I have decided to provide the following info regardless:
For processor upgrades :
Check out Other World Computing(you can return to this link to find info about upgrades for the 300mhz or 500mhz)
When the above webpage loads, select the type of upgrade you need then click the
"Show Upgrades" button:
The next page lists all the possible choices and you can see how much it's going to cost you.
I found this link regarding processor upgrades:
G4 Processor Upgrades
How-To: Upgrade the processor on an older macintosh G4
G4 Sawtooth Overclocking (Optimize your Processor!)
Regarding the Monitor side of things ...
The Sawtooth's Graphics has a VGA
(Video Graphics Array) and a DVI-D
(DVI Digital) display connection so you'll need to check that any monitor you find has the above connectors.
This has some good info regarding the ATI Rage 128/ATI Rage Pro graphics card that is in those model G4's:
Can my G4 handle a widescreen monitor
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@ The Mac:
I'd like to finish by saying that bribing is totally unnecessary and any assistance offered is done because I enjoy volunteering my services to this forum and NOT for any gain, be it a stats boost of sorts in the form of rep/thanks, so whether you choose to do those things is entirely up to you.