Hi I'm an owner of a 435MT as well. Recently I've done a lot of upgrades including a GPU, CPU cooler and a case to rosewill Challenger. I'm currently researching whether it is worth upgrading an i7 920 Intel compatible motherboard, and leaving existing chipset.
If you do decide to proceed with the swap, I highly recommend you consider the following options:
1) You may need to upgrade your case (will allow you more room for CPU cooler, GPU upgrade if you need it, and most importantly you won't likely have an issue putting in the new motherboard, where's you may have big issues putting in an ASUS mobo into the old Dell case etc etc)
2) From what I heard, you will need to purchase a copy of Windows (VISTA/7 whatever you want), since the Dell copy will only work with the existing Dell motherboard.