dmesgの該当部分っぽいところの出力は以下の通りです。
pchb0 at pci0 dev 0 function 0 "SIS 630 Host-PCI" rev 0x31 vga1 at pci1 dev 0 function 0 "SIS 6300" rev 0x31: aperture at 0x48000000, size 0x400000
Section "ServerLayout"
Identifier "XFree86 Configured"
Screen 0 "Screen0" 0 0
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
EndSection
Section "Files"
RgbPath "/usr/X11R6/lib/X11/rgb"
ModulePath "/usr/X11R6/lib/modules"
FontPath "/usr/X11R6/lib/X11/fonts/misc/"
FontPath "/usr/X11R6/lib/X11/fonts/Speedo/"
FontPath "/usr/X11R6/lib/X11/fonts/Type1/"
FontPath "/usr/X11R6/lib/X11/fonts/CID/"
FontPath "/usr/X11R6/lib/X11/fonts/75dpi/"
FontPath "/usr/X11R6/lib/X11/fonts/100dpi/"
EndSection
Section "Module"
Load "dbe"
Load "extmod"
Load "glx"
Load "record"
Load "xtrap"
Load "speedo"
Load "type1"
EndSection
Section "InputDevice"
Identifier "Keyboard0"
Driver "keyboard"
EndSection
Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "wsmouse"
Option "Device" "/dev/wsmouse"
EndSection
Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "Monitor Model"
EndSection
Section "Device"
### Available Driver options are:-
### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
### <string>: "String", <freq>: "<f> Hz/kHz/MHz"
### [arg]: arg optional
#Option "SWcursor" # [<bool>]
#Option "HWcursor" # [<bool>]
#Option "PciRetry" # [<bool>]
#Option "rgbbits" # <i>
#Option "NoAccel" # [<bool>]
#Option "TurboQueue" # [<bool>]
#Option "SetMClk" # <freq>
#Option "FastVram" # [<bool>]
#Option "ForceCRT2Type" # [<str>]
#Option "ShadowFB" # [<bool>]
#Option "Rotate" # [<str>]
#Option "NoXvideo" # [<bool>]
#Option "Vesa" # [<bool>]
#Option "MaxXFBMem" # <i>
Identifier "Card0"
#Driver "sis"
Driver "vesa"
VendorName "SiS"
BoardName "630"
BusID "PCI:1:0:0"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 16
SubSection "Display"
Depth 1
EndSubSection
SubSection "Display"
Depth 4
EndSubSection
SubSection "Display"
Depth 8
EndSubSection
SubSection "Display"
Depth 15
EndSubSection
SubSection "Display"
Depth 16
Modes "1024x768"
EndSubSection
SubSection "Display"
Depth 24
EndSubSection
EndSection
XFree86 -configureしてできたXF86Config.newの"Device" Sectionの Driverを"sis"から"vesa"に書き換えただけです。また、wsmoucedを使っ ているので念のため/etc/rc.conf.localに以下を追加しておくと良いか もしれません。
wsmoused_flags="" # for ps/2 or usb mice: "", serial: "-p /dev/cua00"