MSI Radeon 9800 Pro With Dell Dimension 4500?

Hello all, first and foremost, I'm quite sorry if this has already been asked... However, I would like an individual response.

OK, I have a Dell Dimension 4500.
Pentium 4 2.4 GHz
512 MB RAM
GeForce 4 MX 420

I'm trying to keep up with the PC gaming and I will tell you this, it isn't very fun knowing the only way you can play Doom III is on 640X480 on low settings with all special effects off...

With that said, I would like to upgrade my video card from the lacking MX 420 to a 9800 Pro, specifically the MSI 9800 Pro from Newegg.

Here's the link if you're interested.
http://www.newegg.com/app/viewProductDesc.asp?description=14-127-127&depa=0

Now I know my PSU is at 250 watts, which is below the requirements of this particular card. However, I have heard that Dell 'underrates' their PSU's and that the Power Supply in my Dimension 4500 peaks out at about 345 Watts. Can anyone verify if that particular 9800 Pro will run without flaws on my system? The last thing I want to do is spend 200 bucks to get the card then try to install it and something goes horribly wrong... I really want to go for the 9800 Pro as I have heard that my second choice, the 9600 XT, is a huge step below of the 9800 Pro. It would be safer, but you cannot even compare the performance of a 9600 XT to a 9800 Pro.

Here's the link to the 9600 XT.
http://www.newegg.com/app/ViewProductDesc.asp?description=14-131-232&depa=0

So can anyone verify that the MSI 9800 Pro will be OK with my Dell Dimension 4500? If anyone has this card already, did you have any problems installing? Mainly was there anything different you had to do when installing this video card, as compared to other video cards? Thanks in advance!