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In order to know, we need to know all specs on your computer including HDD's, optical drives, USB peripherals, etc. All of these draw power and 450 is a bare minimum for a high power card like the 1900XT
According to the link you gave, the 450Watt supplies 26Amps on the 12V rail. I am not exactly sure what the requirements are for the X1900 XT, but I do know that for the X1900 XTX ATI suggests 30A for a typical system. I have a Antec 380 2.0 PS and it puts out 32A on the 12v rail (actually, 16A on each of two rails). I have 3 HD's and 1 Optical as well as (5) 120mm fans and I have no problems with it and my X1900 XTX.
BTW: From what I saw, the 530Watt PS (the same 'Modstream' series) 'only' offers 2 amps more on the 12 rail. That would still leave you short of what ATI recommends if the requirements for the 1900 XT are the same as the 1900 XTX.
Imo the modstream is a bit long in the tooth & has never had the power of the 520 Powerstream. I'd look to upgrade
As u can see Power Consumption and Noise Level of Contemporary Graphics Cards pulls around 120w=12A on load the average for other components is probably between 20~24A giving 30~36A probable load.
What to upgrade to depends on your further plans if u might go crossfire at some time go 700W (4x18A) if not you can manage with a decent 550+ I'm more than happy with the 1 in my sig.