If you look at the http://ppbm4.com/Benchmark.html results, you will see several similar systems with scores between 110 and 130 seconds. With your 4 disk raid0 you may do even better. You can try it yourself.
For DV and HDV that is perfectly OK. Now this is with CS4. My own experience with CS5 is that it is significantly faster.
Without MPE, my score improved from 38.0 to 23.3 seconds just by the move from CS4 to CS5 on the exact same system and under the same conditions. If one assumes a similar performance increase on your system, a 110 second score with CS4 would improve to around 67 - 68 seconds. That is as respectable score.
Whether the 250 can be "hacked" to enable MPE, I do not know.
To summarize, you are OK for DV and HDV with CS4 and with CS5 you are even better equipped. No need to invest in new hardware.
Harm it seems you never sleep thanks this helps alot ,I did find this which may help some of the poeple that feel they may want to do a small upgrade with just there graphics card gtx285 the person is a forum user on the Adobe Forum he called Wrigley and he tests the a older or Avgrage system and it even seem to runn ADCHV fine and HDV smoothly