This is a bit of a regurgitation of the answer I gave not to long ago to Captain Planet.
As a new to Mac and OS X user, your main concerns should be hardware and OS stability. I would tell anyone wanting to buy their first Mac to get it now rather than wait for Leopard for 2 reasons:
1. No one knows how long till Leopard yet and most things point to May/June not April and maybe not till late May at the earliest.
2. The first release of a new OS can be buggy, so you may want to get your iMac now or soon and then once you know and love it, maybe when there is a 10.5.2 or 10.5.3 release, sometime around end of summer, spend the $129 for the OS upgrade.
However, that said, the possiblity of an iMac redesign throws a kink into that. Again, we don't know what is coming, definitely not when, and most likely beyond your time frame. Also, as has been seen with the MacBooks and other new releases, rev. A's of a product can sometimes have issues that early adopters have to deal with. I usually don't recommend a switcher be an early adopter. Now the iMac may still have most of the same guts and just a cosmetic change which means that it may not have those issues.
Lastly, once there are new iMacs, you can probably get the current design from MacZone or MacMall, CompUSA, etc. at a $200-300 discount or refurbs (which are great deals and great Q.C.) however, neither resellers or refurbs get you upgraded graphics card or any other enhancements but RAM. And I think you definitely want the graphics and RAM upgrades, possibly a HDD, I would go for the 500GB over the 250GB.
All that said, I would buy now, or wait till mid April to see if Leopard is either announced or new iMacs suddenly appear. I'm guess not though since new iMacs will mostly likely feature the Santa Rosa chipset which Intel won't be releasing till May. Then again if you really want the new design and you're counting on there being one, wait and wait and wait...