I have a VIA P4M266 motherboard (with intergrated graphics) that runs a Celeron 2.6 GHz processor (478 socket). I use engineering applications, namely PTC's ProEngineer, which is quite graphics intensive. I therefore purchased a graphics card (radeon 9550 256mb AGP 8X), but much to my dismay the motherboard only supports 4X Agp. Of course everything runs still. Further investigation of my motherboard's specs revealed that it is only DDR266 compatible, but I am using DDR400 Ram (768 MB).

My Question is this : How much performance am I sacrificing by using this motherboard?
And is worth upgrading to a 8X AGP & DDR400 compatible board?

I am aware that the 478 socket boards are being phased out, but they are still Available in South Africa for about $50 (R 350).
What should i consider compatibility wise, when buying a new motherboard - power supplies, connector cables, tower case size?

Any advice Is much appreciated