You need the xf86-video-intel driver package for X11:

# emerge xf86-video-intel

In /etc/X11/xorg.conf go to section "Device" and change the driver from "vesa" to "intel".

With my setup (kernel 2.6.30-gentoo-r4) 3D acceleration and DRI worked out of the box. If you have problems, check the following.

You need MTRR support in your kernel:

Processor type and features  -->
[*] MTRR (Memory Type Range Register) support 

You also have to enable AGP and support for the Intel GM45 Express chipset using the Intel I965 driver:

Device drivers  -->
  Graphics support  -->
  <*> /dev/agpgart (AGP) Support  -->
      ...
      <*> Intel 440LX/BX/GX, I8xx and E7x05 chipset support
      ...

You can safely unselect support for the other drivers here. You also have to check the kernel support for DRI:

Device drivers  -->
  Graphics support  -->
  <*>  Direct Rendering Manager (XFree86 4.1.0 and higher DRI support) -->
      ...
      <*> Intel 830M, 845G, 852GM, 855GM, 865G (i915 driver)  --->
          ( ) i830 driver
          (X) i915 driver 
      ...

Again, you can safely unselect support for all other drivers.

If you have changed something, re-compile and re-install the kernel and check that the kernel detected your hardware:

$ dmesg | grep agpgart
[    0.422080] Linux agpgart interface v0.103
[    0.422201] agpgart-intel 0000:00:00.0: Intel Mobile Intel® GM45 Express Chipset
[    0.423241] agpgart-intel 0000:00:00.0: detected 32764K stolen memory
[    0.426290] agpgart-intel 0000:00:00.0: AGP aperture is 256M @ 0xe0000000

In /etc/X11/xorg.conf you must load the "glx" and "dri" modules:

Section "Module"
        ...
        Load   "dri"
        ...
        Load   "glx"
        ...
EndSection
...
Section "DRI"
        Mode 0666
EndSection

Restart X if you had to change your configuration. You can check that everything works with the program glxinfo

# emerge mesa-progs
# exit
$ glxinfo | grep '^direct rendering'
direct rendering: Yes

With glxgears you can see DRI in action.