I dont know what industry you are in, or how competetive Apple are on price in your country. I have only given my point of view in comparison of the iMac to my high end gaming PC. I dont know if the iMac's 3D performance would compare, I can say that it's desktop/application performance is very good indeed.
Fogetting that the default install of OSX uses a bucket load of it and that 512mb isn't really enough, if you reinstall OSX or you are putting Windows on it that becomes neither here nor there. I would personally choose to upgrade the Intel iMac's RAM to 1gb, possibly 2gb, but unless your buying so charged RAM it is so cheap that this is a moot point.
In the UK an iMac with 1gb costs just over £1k including taxes that a business would get back, running in a Windows environment it will perform much higher than it's advertised clock speed because the iMac architecture is genuinely much quicker than a conventional PC - I did a benchmark test on one yesterday that astounded me when I compared it to my PC, the iMac was much faster.
What this means is there's some tasks the iMac will do quicker than my PC which cost £100 more before I bought the screen for it. However the iMac would loose out heavily doing 3D rendering and be a bit slower doing video work ... so it's all down to what you need the computer for as to whether it will perform well enough.
With regards warranty the iMac comes with a 1yr warranty which is comparable to most of the bits inside my custom built PC - my RAID array is warrantied for longer. Apple's are a very reliable brand, so it's 6 of one and half a dozen of the other.
You're talking about the Mac in relation to business, in which case if it's mission critical a warranty doesn't cut it anyway - and neither does skimping on price. If it's not mission critical then you still either want contracted support or an office backup machine so you aren't left hanging if something does go wrong - which historically with Apple there's very little chance of - wouldn't the cheap base spec iMac serve as an excellent backup machine if the worst happens?
If it's mission critical then forget moaning about price, if you skimp on a mission critical machine and dont invest in redundancy then you deserve bankruptcy.
I'm not saying the iMac is better, I think they look far too ugly (designed by a 7 year old) to be put anywhere near my home office. I'm just saying it is a serious contender on spec and price when I compare it to my PC. The performance of the thing really does fly, and even with their lowly RAM small Hard Disk base specification they punch a heck of a weight for the price.