So here is it. I am having p4 2.8ghz, 1.5gb ram and would be buying 8800gs.  
I don't want 99fps in games and around 50fps will do and 25-30 in crysis. I play at 1024x768.
Should I upgrade my processor to e2140 (ocing around 2ghz) ?
I won't unless it's absolute necessity.

Check if your old mobo can really support E2140, my story on Dell E510 of P4 630 (3 GHZ) of 945G chipset was not available to support Core 2 Duo nor Core 2 Quad, so I used HD3870 on my Dell E510 with P4 630 for half year with only half performance of 3870 . So let's check if your mobo can support E2140 before you buy it.

good ideam ull never get 20 fps in crysis with a 2.8 p4, unless u play on low or 800x640

The 8800gs will be severely bottlenecked by p4, and 8800gs is a pretty slow card to begin with. You'll be lucky to get 15fps in Crysis at that resolution.

Yes, I totally agree Dagger, on the CPU comment.  But the 8800GS is actually quite capable in Crysis, especially at 10x7.  
 
I've been testing an 8800GS in many games paired along with 3 CPU's over the past few weeks.  Here are a couple of my performance charts showing just how bad a single core A64 4000+ holds back the 8800GS in parts of Crysis.  Obviously an A64 4000+ > P4 2.8GHz.
 
Medium Details:

 
High Details:

 
 
Notice in this test, with the X2 5600+, the 8800GS can average 56 fps at 8x6 medium, yet is held to under 30 fps with the A64 4000+.  And at high details 47 fps with the 5600+ is knocked down to 22 fps. I ran 4 different levels, 2 more cpu oriented like above, two more GPU oriented.  At playable resolutions, the single core A64 impacted performance in all cases. 16x12 was often too much for the 580/1400 factory clocked XFX 8800GS.  The evga superclock at 650/1900 would obviously do better.  
 
I'll get around to posting more results soon in a seperate thread.  These are just two of my 10 pages of Crysis charts alone.  Also did many other games.  

That's some interesting data you got there pauldh.  I'm suprised how well the X2 4200 did.  Not too far off the top chip.

Thanks pauldh for the chart. But now imageshack isn't loading .
I agree that crysis is cpu+gpu intensive game. But what for the likes of oblivion and assassin's creed ? or say bioshock ? As I said , I don't aim for 99fps and am ready for even 800x600 because my monitor is only 15"  so can I get =50 fps in other games ?

Hey Thx.  You'll need to see all the Crysis charts to get a better picture.  But with the CPU handling the physics, Crysis sure crushes the single core CPU.  
 
The X2 4200, kept it close when the GPU started to become the limiting factor. 16x12 medium and 12x10 & 16x12 high for example.  All in all, the 4200+ did pretty decent in Crysis.  
 
In other games it lagged behind the 5600+, especially Oblivion.  Oblivion fascinates the daylights out of me when you explore various areas of the game at lots of settings.  I've got all the data in excel, just no time to organize the charts and write up a summary.  

imageshack won't load already?  That was quick and has been one reason for my delay in posting charts.  (btw, it still loads for me)
 
The thing is resolution wont' hurt you when the CPU is the limiting factor.  Detail levels will hurt you.  Oblivion, until you stress the GPU, you could play at 12x10 just as well as 800x600.  One thing I found with Oblivion, the minimum fps with the single core CPU suffered even more than the average fps.  Another thing, the XFX 8800GS was actually quite playable at 12x10 max details 2xaa/16xaf.  16x12 max details 0xaa/16xaf was also playable.  
 
The most GPU demanding area  is the Outdoor foliage areas (waist high grass) in the 3rd person view.   I actually did all testing in 3rd person view as fps are lower that way.  Did a few comparisons tests first vs third person for fun.    In Towns, the CPU was greatly stressed,  and resolution had little affect on performance.  
 
At 800x600 it will totally be your CPU holding back the performance.  The 8800GS has no problems at that res.   Crysis will hurt your CPU.  Oblivion, you'll suffer some low drops in some areas unless you turn down some detail levels from max.   Farcry and fear are no problem for single core.  Need for speed carbon and testdrive unlimited were ok on the A64 4000+, but I know that An Athlon XP 2800+ struggles bigtime in Test drive unlimited even at low details.  I had an A64 3000+ I was going to test also, but the time involved for each CPU was massive, so I left out the 3000+.  Too bad , it would have been closer to your cpu.  

At last could load the image through php proxy.
It seems there's no other choice than upgrade my cpu.  
Am thinking of e2140/60, Is there any cheap mobo ($50) for moderate overclocking of dual core e2160 (-> 2.6ghz) ?? is nvidia 7050/610i gud for this??

Well, to be honest, you could upgrade your GPU, play your games, and see if you are happy.  If not look to make the bigger mobo/cpu plunge.  Since you can use a PCI-e card now, you don't have to do the upgrade all at once.  
 
BTW, the $110 evga 8800GS superclock blows out this card I used.  At your 10x7 or 8x6 resolution, you would not need to spend more on the GPU.  I really like the 8800GS up to 12x10 or 14x9 gaming.  Price is fabulous.  

 I think some of the newer E510 chipsets do support Core 2 duo. If you bought it when it had the Pentium 4, then that was a while ago. I bought mine when its life was about to be over and Im pretty sure it supports Core 2 Duo. I'll have to dig up some stuff first.