Boinc no longer sees Nvidia GPU after update, Mint 17.

Not sure why you are having problems but I am running 331.38 on Ubuntu 14.04 with a GTX770:

uname -a
Linux node-name 3.13.0-40-generic #69-Ubuntu SMP Thu Nov 13 17:53:56 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

nvidia-smi -a

==============NVSMI LOG==============

Timestamp : Sat Dec 13 19:01:29 2014
Driver Version : 331.38

Attached GPUs : 1
GPU 0000:01:00.0
Product Name : GeForce GTX 770
Display Mode : N/A
Display Active : N/A
Persistence Mode : Disabled
Accounting Mode : N/A
Accounting Mode Buffer Size : N/A
Driver Model
Current : N/A
Pending : N/A
Serial Number : N/A
GPU UUID : GPU-2da07952-e162-bde3-48ac-28daf832b2ff
Minor Number : 0
VBIOS Version : 80.04.B4.00.01
Inforom Version
Image Version : N/A
OEM Object : N/A
ECC Object : N/A
Power Management Object : N/A
GPU Operation Mode
Current : N/A
Pending : N/A
PCI
Bus : 0x01
Device : 0x00
Domain : 0x0000
Device Id : 0x118410DE
Bus Id : 0000:01:00.0
Sub System Id : 0x103310DE
GPU Link Info
PCIe Generation
Max : N/A
Current : N/A
Link Width
Max : N/A
Current : N/A
Bridge Chip
Type : N/A
Firmware : N/A
Fan Speed : 81 %
Performance State : N/A
Clocks Throttle Reasons : N/A
FB Memory Usage
Total : 2047 MiB
Used : 861 MiB
Free : 1186 MiB
BAR1 Memory Usage
Total : N/A
Used : N/A
Free : N/A
Compute Mode : Default
Utilization
Gpu : N/A
Memory : N/A
Ecc Mode
Current : N/A
Pending : N/A
ECC Errors
Volatile
Single Bit
Device Memory : N/A
Register File : N/A
L1 Cache : N/A
L2 Cache : N/A
Texture Memory : N/A
Total : N/A
Double Bit
Device Memory : N/A
Register File : N/A
L1 Cache : N/A
L2 Cache : N/A
Texture Memory : N/A
Total : N/A
Aggregate
Single Bit
Device Memory : N/A
Register File : N/A
L1 Cache : N/A
L2 Cache : N/A
Texture Memory : N/A
Total : N/A
Double Bit
Device Memory : N/A
Register File : N/A
L1 Cache : N/A
L2 Cache : N/A
Texture Memory : N/A
Total : N/A
Retired Pages
Single Bit ECC : N/A
Double Bit ECC : N/A
Pending : N/A
Temperature
Gpu : 60 C
Power Readings
Power Management : N/A
Power Draw : N/A
Power Limit : N/A
Default Power Limit : N/A
Enforced Power Limit : N/A
Min Power Limit : N/A
Max Power Limit : N/A
Clocks
Graphics : N/A
SM : N/A
Memory : N/A
Applications Clocks
Graphics : N/A
Memory : N/A
Default Applications Clocks
Graphics : N/A
Memory : N/A
Max Clocks
Graphics : N/A
SM : N/A
Memory : N/A
Compute Processes : N/A

Did you try to restart the client? sudo service boinc-client restart

Sorry, sometimes numbers (and apparently decimal points) confuse me. :>)