Get it. Don't buy the expansion pack yet unless you get it for a killer deal price. DON'T install the expansion pack yet.
DO install Rome Total Realism. It's a player-made modification that makes the game much, much better. The only issue you may run into is the new RTW cds might have upgraded to version 1.5 whereas RTR is designed for 1.2.
There's good news for you if you do find that it is 1.5. First of all, there is a "Rome Total Realism Platinum Edition" which is for it, but which was not primarily designed by the Rome Total Realism team (They did the "Gold" edition). I cannot say if this is or is not buggy as hell. I can tell you the "Gold" edition for 1.2 is generally not.
More good news though - the RTR team is working on RTR 7.0 which will be for the Barbarian Invasion Expansion pack. Expect bugs at first, but also expect them to be resolved promptly. This team really is dedicated.
When 7.0 will be out, I can't say. But, hopefully, within the year (and once you try Gold, you'll wait for it as if it were CT/TOD).
You can find the download for Gold at totalrealism.net
You can find other downloads, such as platinum, at various other RTW sites.
Word of warning - do not visit rometotalrealism.com. There was a schism within the group and this site is outdated, and possibly a danger to your PC.
I hope this helped.