Might want to take the Daring Outlaw feat from Complete Scoundrel.
Makes your Swashbuckler and Rogue levels stack for the purposes of determining Grace, Swashbuckler Dodge bonus, and Sneak Attack dice. So you'd gain +1 Dodge AC and +1d6 Sneak attack (As if you were a level 6 Rogue for Sneak Attack and a level 6 Swashbuckler for Grace and Dodge AC).