512MB links? Wow... they are incredibly fast! I'm guessing you meant to say 512Kbps links!
Since you are running Server 2003 R2 edition and Server 2008, the DFS configuration which you will configure is pretty much compatible. I have a client which has such a configuration over an even slower and unreliable 256Kbps DSL line to their branch office, and it works with no problems. I can store a 200MB file on the file server and have it in the branch office by morning, when demand is low.
Compared with DFS in the pre-R2 edition of Server 2003, which would instead use FRS, DFS between Server 2003 R2 and Server 2008 is much better, much improved and should cause no problems for you whatsoever. The most important part is to get your replication topology correct. I also suggest that if there are is any data which doesn't have to be replicated immediately between sites, then you schedule that for nightly replication. As an example, my Share which stores Software Applications, GPO Packages and so forth is configured to only replicate after hours, starting about 8 or 10pm and running until 6am in the morning.
-tigermatt
