Hi thebestrc, and welcome to AF
Apologies, this is long, but I hope will help.
HTC Sync only works with ROMs based on HTC Sense software. So if you want to use that then you will need a Sense ROM.
In principle changing ROM should be straightforward, but there are a few steps, a couple of things to watch for, and it's best to make sure you understand what's needed before trying to do it. So the starting point is the sticky thread Desire All Things Root Guide in this forum. That contains links to a number of useful threads, and I suggest reading "[FAQ] Rooting" first - you are rooted, but that includes a lot of other info, including the basics of ROM flashing.
The most important thing is taking backups before changing anything. If you have apps and data you wish to keep then I recommend Titanium Backup (esp the paid "Pro" version). When you change ROMs you must perform a factory reset, which erases everything. An app like that backs up your data to sd, and you can then restore them to the new ROM. The Pro version lets you do batch restores, and also backs up call logs and messages in a way that can be restored between ROMs (otherwise you want an extra app to back these up). Contacts should be backed up to sd (import/export) or to Google.
Then before changing anything, boot into recovery (boot while pressing the volume down key, then select recovery from the menu) and take a backup from there. This is a "nandroid" backup, which lets you restore your ROM and all data if something goes wrong.
That all done, you can think about changing ROM
. The faq outlines the procedure, and the All Things Root Guide post also has a link to a list of ROMs at Xda. Do read the first couple of posts of a ROM's xda thread before flashing to see whether there are any special requirements. I think HTC Sync will work with any Sense ROM, but those using Sense 2 or later will need an extra partition on your sd card. If the card came with the phone you probably have one. Install an app called Quick System Info and it will tell you ("a2sd storage" ). Titanium will show this too (bottom of main page).
One thing to watch: have the internal storage partitions been modified? That would affect what ROMs you can flash. Boot while pressing the volume down key and you get a white screen with a menu. This is the hboot screen. What does it say at the top? In particular, is there any pink writing, and if so what does it say? If there is none, or if it just says Revolutionary or "Bravo Stock" you have standard storage. Otherwise tell us what it says, and do not try to flash a simple Sense ROM (or run an RUU).
Now all of that is about changing ROM. The alternative is not to use HTC Sync - I think very few of us here do. There are 3rd party PC sync utilities which don't need HTC software - can't remember names offhand but a search of the Android Applications or Android Lounge forum here would find them (am typing on phone while walking so can't easily dig up links now). Or you may be able to manage without - transfer contacts to your Gmail, just drag and drop media to the phone, etc. It depends what your requirements are.
Good luck - hope that's not too exhausting to read!