Do you currently have a 9800GTX+?
If so, you can try the following:
1) Use your 9800GTX+ until you get more money. Yes, the 4890 is noticeably faster, but not very much. Probably around 15-20% faster. Which is not a whole lot, considering how much money you will spend on a new card. You may want to check out benchmarks to see how big the difference really is.
2) Sell your 9800GTX+ and use the extra cash to get a HD 5850. They were going for $270 or so a few weeks ago on newegg.com.
3)
You can give AMD Phenom 2s a try. Benchmark between Intel 920 and Phenom 2 965. The AMD Phenom 2 965 is a little slower than the Intel Core I7 920, but costs around $100 less at newegg.com. Look at the performance, is is not much slower.
So if you save $90-100, you can put that towards the 5850. That would make a faster overall PC for games.
Costs about $30 more than the Q8400, but as you can see, the Phenom 2 965 is faster than the Q9550. Which means it will most likely be considerably faster than the Q8400.
I think with the AMD CPU, you will have enough cash for a HD 5850 and get an overall more powerful PC.
Seems to be the better price/performance deal out of your Intel options. Plus, LGS 775 is dead. AMD3 socket is newer and will be around longer. They might eve release new CPUs in AMD, meaning you would be able to upgrade again. Can't do that with Intel.
I am happy with my AMD Phenom 1 9950 quad core, and this quite a bit faster.
Also, why are you going with a 32bit OS? You're just limiting yourself. You can't even use 4GB of RAM, and more programs in the future will use it.
I have been running Vista 64bit for around a year with almost zero issues.