[G4] keep g4 or upgrade to g5?

[YOUR FRIEND, CHRIS]

Greetings!

i have a 2000 g4 450dp. i'm on a tight budget {obviously!} and trying 
to determine if i would be better off by upgrading it {ram, video card, 
2nd hdd - and possibly processor}

OR

purchasing a dual 2.0 g5. the machine i'm looking at is number: 
M9032LL/A

http://support.apple.com/specs/powermac/Power_Mac_G5.html

and has a radeon 9800 pro 128 card, 4.5gb ram, 250gb hd and a firewire 
400 card; 10.3.x

can anybody PLEASE advise me:

1- is this a good unit to choose; reliable, lack of issues, bad 
components etc

2- is this a wise economic decision versus uprgrading the g4; most 
importantly in so far as being able to carry me into the future for a 
good long while as the g4 has {8 years!} - and do these projected 
longevity gains, as well as in performance increase, justify the price 
difference over upgrading? as i see it, all i'll have to do is add 
another hdd to it which is inexpensive.
i'm primarily using it for internet {ebay, burning cd's, music, videos} 
and very light photo editing. i just bought a copy of tiger; i've read 
here of some dual g5's having problems with leopard. any thoughts?

3- what is a fair price for this unit? the seller is waiting for me to 
make an offer; if it is within my price range, and based on feedback 
here a good unit i'll grab it!

many many thanks in advance!

chris