deathspeed - FS_Open is definitly going to kill your system.
Although the old engine was the base of the development, Bobbau, Sticks and everyone else in the SCP team I'm blatant to list here (although they were cruical in development) pulled off a terrific development (for Intergayat least, if they ever took notice of it, they wonder why they ever fired :V: and/or not hired the team).
They moved the whole engine to DirectX 8 from 5, introducing a wide variety of features and enhanced modder friendliness. HT&L (Hardware Transformation and Lighting - ergo the vidcard makes the whole thing run tons faster) switch resulted in a huge performance gain.
Then a lot of high-end features and effects were introduced, that put even recent games to shame - the size of textures, the number of polygones -, so to get the full out of the developments you'll need a high-end system.
To sum this whole SCP worshipping post up - buy a good videocard, one that fully supports DirectX-9.
Hopefully that conversion will took place within a year.