Well isnt that interesting.

The exact same thing happens to me at the exact same points in the game!
When releasing Grunt and when Samara confronts Morinth.
But I have been trying to solve this problem for about 2 months now and so far no luck.
My System is nearly the same as yours.
-Intel Core2Duo E6600
-2 gigabyte RAM
-Asus P5B Deluxe Mobo
-Sapphire Radeon HD 4870 1gig
-Creative XF-i Titanium
-Windows XP Home SP3
-Trust 570W Dual Fan PSU
-------------------------
Now to what I tried to find and solve that Problem.
1. I switched the ATI Catalyst drivers through. From 9.3 to 10.3 -> No changes, error still appears
2. I took out my Soundblaster, uninstalled all its drivers and used the on board sound -> No changes, error still appears
3. I reinstalled Windows with SP3 and the Game only (No other drivers except ATI and Direct X, No CCC, soundblaster etc.) Game runs with minimum settings. Vanilla Windows no internet, no anti Virus or anything which can interfere. -> No Changes, error still appears
4. Tested my RAm with Memtest86+ in Dos mode for 6 hours straight -> not one error
5. switched from dual channel to single channel -> no changes, error still appears
6. Updated BIOS from graphicscard -> no changes, error still appears
7. sent the graphics card back to Sapphire
twice! Came back with the report it is 100% working and no error could be discovered! -> tried using it afterwards -> guess what? Same error again!
8.And now I thought, okay! Maybe it is Mass Effect 2. Installed the patch when it came out. -> No changes, error still appears.
9. tried GTA4, GRID, Mirrors Edge, Fallout3, Oblivion, NFS 4 High Stakes, GTA San Andreas, Far Cry 1 + 2, Crysis, Assassins Creed 1, NFS Shift, Undercover and Pro Street, Juiced 1+ 2 and even Half Life 1 to HL2:Episode 2!
All those games crash! Some at a certain point, others completely random. The error is always the same one.
And I have it right here:
It is a
ATI2mtag Error. ATI has no Idea what this error causes and what can be done.
---------------------------------------------------
Ereignistyp: Fehler
Ereignisquelle: ati2mtag
Ereigniskategorie: DAL
Ereigniskennung: 43038
Datum: 30.03.2010
Zeit: 17:01:37
Benutzer: Nicht zutreffend
Computer: DJ
Beschreibung:
EDID contain an error in the RangeLimit field
Daten:
0000: 00 00 00 00 01 00 5a 00 ......Z.
0008: 2a 00 00 00 1e a8 00 c0 *....¨.À
0010: 00 00 00 00 00 00 00 00 ........
0018: 00 00 00 00 00 00 00 00 ........
0020: 00 00 00 00 00 00 00 00 ........
------------------------------------------------------------------
After searching all around the internet about this error, I found out that every ATI graphics card, beginning with the HD series, has this error and ATI cannot figure it out!
After installing my ATI x1950 PRO, the error dissapeared partly. Windows still gave me this error message in the Systemlog, but the PC did not reboot.Funny, cause my X1950 has a faulty 3D accelerator.

I found out, that there are 2 possible causes for this error:
1. The PSU can´t give you enough power
or
2. The graphics card is faulty
Strangely enough, it appears to be a gamble what may solve this issue.
The ones I have found are:
1. Reinstalling Windows
2. beeing one of the lucky ones which dont have that error
3. buying a new PSU
4. buy an NVIDIA! They somehow managed to eliminate this error shortly after it appeard with geforce cards. Since then it never appeared again!

I wrote an e-mail to Sapphire about this issue already and I´m still waiting for an answer. Reports show they sure take their time in answering.
My take on this is, that it is the graphics card, or it really is the power supply. Latter does seem not to be the case though, cause apart from these two points, the game runs without any error.